This chef's sumptuous creations are almost too pretty to eat

''The question is, will it fly?'' says Eli Alperowicz, using his paring knife to carve a cardinal out of a large beet. He holds the bird in front of him to study it, and with a few more strokes gives it a dignified crest and graceful tail. Then he puts the cardinal on its bamboo skewer perch.

Eli Alperowicz is a chef who specializes in fancy garnishes. With sleight of hand he turns improbable vegetables into exquisite flora and fauna. Beside the cardinal, a basket of flowers made out of turnips, radishes, carrots, and onions adorns the counter where he has been working.

''Now, how do you think I will make the cardinal's cage?'' he continues. Taking a honeydew melon, he first slices a thin piece off the bottom to give the melon a stable base. Then he puts his paring knife in at midline.

''You must remember where the middle is and always come back to it with each scallop,'' he explains, quickly circling all the way around the melon's circumference in even arcs. He pulls the two halves apart, inserts skewers around the scalloped rim, and puts the canopy top onto the open ends of the skewers.

The brilliant beet-red cardinal perched in its pale green honeydew cage is one of several table decorations Alperowicz has suggested for Christmas. As a garde manger chef, who specializes in the cold buffet table where pates, cold meat and fish dishes, fancy salads, and vegetable garnishes are presented, he is full of ideas for holiday ornaments made out of food.

Dressed in his chef's toque and whites at this demonstration at Newbury Junior College, where he teaches garde manger, Mr. Alperowicz speaks directly to his audience about his metier: ''The major thing is time and patience.'' To encourage those less practiced than he is, he says that no matter how skillful, he must still watch closely and take his time. Rushing will only ruin whatever he is making.

He refers to his knives and other tools as ''my study kit.'' Besides the razor-sharp paring knives and thin, curved tourner knife for fluting mushrooms and other round surfaces, Alperowicz has a wide assortment of implements.

Although traditionalists might raise their eyebrows, he says he uses any tool that creates an interesting effect. This includes a surgical knife and other medical instruments, a wood-carving knife, wood file, and various carpentry tools.

As Alperowicz shows how to shape a gardenful of flowers from vegetables, he gives tips along the way. To make unpeeled vegetables more malleable, for instance, he suggests soaking them in heavily salted water with about 7 percent salt. After he completes each individual flower, he drops it in ice water, where it opens up and ''blooms.''

With a woodcarving tool, Alperowicz fashions a red chrysanthemum out of a Spanish onion. ''It's not very romantic,'' he comments on the beautiful flower. ''You wouldn't give this to a loved one.''

From a large carrot he slices a long, thin rectangle in which he cuts lengthwise slots. When folded in half and attached with others to a toothpick, the carrots become a spectacular blossom.

Taking a yellow or red pepper, he shows how to cut diagonally from near the stem down to the tip, making long pointed petals, with the seeds on the inside appearing as the flower's center. After forming petals around a radish, he cuts away on the inside to make the white center contrast with the red petals.

Unlikely beet stems, trimmed and nicked at intervals on an angle, miraculously shape themselves into graceful foliage when immersed in the cold water. The beet root itself he sculptures into a gorgeous rose.

Taking a leek in his hand, he slices vertically almost through to the root many times, then snips away at the unruly top. ''You give it a haircut,'' he says. In cold water the floppy leek eventually curls into an exotic flower.

Using skewers as stems, Alperowicz arranges a bouquet in the basket. ''It's not more expensive than a big flower arrangement,'' he says, ''and afterward you can make soup out of it.''

Eli Alperowicz was born in Lithuania but grew up in Israel. Although trained in electrical engineering, he later decided that field was not for him.

''Food was very important in my house,'' he explains. ''When I was growing up I constantly watched my mother prepare meals for the family. Everybody says his mother's a great cook; my mother really was a great cook.''

So at the age of 28 he went to Tadmor, a professional chef school in Israel, for 20 months of intensive training 12 hours a day. He read every cookbook he could get hold of and immersed himself in his new career.

Since then Mr. Alperowicz, who is married to an American and speaks five languages, has worked in restaurants in Israel, France, Canada, and the United States. Before Newbury he was the head garde manger chef at the Hotel Meridien in Boston.

''You know what I learned working in France?'' he asks. ''We in the US have nothing to be ashamed of.'' Then, questioned about American cuisine, he replies, ''I think it has a lot of potential if people won't be pretentious about it.''

On another visit to his kitchen, Alperowicz was busy preparing for a special banquet of American cuisine. For over a month he had been making pheasant centerpieces of salt dough, but these were not for the amateur. Flattening pieces of dough with a rolling pin, he wrapped them around a wire mesh base layer upon layer, cutting, filing, sanding the surface with his assortment of tools to make the dough look like feathers.

After the birds had dried naturally in the air, he was painting them one by one with caramelized cornstarch and rubbing alcohol to give a warm brown coloring. By brushing the color on or rubbing it with his finger, he showed how he could achieve special textures and shading. Each pheasant, strikingly realistic, had its own character.

On a separate table at the side of his kitchen stood a grand turkey prepared for the cold-buffet table at this banquet. It was made of tallow, a combination of suet, paraffin, and beeswax, and sculptured and etched in great detail.

While this regal bird surveyed the room in full splendor, Alperowicz helped a young student who was struggling with mayonnaise. ''Life is a messy job,'' he exclaimed. Then, in a gentle voice he coaxed her, ''We've got to get it fixed today, right?''

Mr. Alperowicz has created the following recipe for the holiday season ahead. It is a white seafood mousse, colored with spinach, pimento, and olives, cleverly constructed so that each slice down the length of the terrine will reveal the glorious Christmas tree inside.

You will find it easier to make than you suspect, a delicious recipe for Christmas entertaining and a delight to behold. Christmas Tree Mousse 1 1/2 pounds lemon sole 1/2 pound scallops 6 egg whites 2 tablespoons lemon juice, or to taste 2 teaspoons salt, or to taste 1/4 teaspoon white pepper, or to taste 1 cup heavy cream 3 ounces fresh trimmed spinach leaves 1/4 cup loose parsley leaves 1 envelope unflavored gelatin Pimento strips 5 black pitted olives

Cut sole into pieces and put in work bowl of food processor fitted with metal blade. Process until sole is completely smooth, turning machine off and scraping down sides with spatula several times. Remove sole to a bowl and repeat with scallops. Add pureed scallops to sole and stir to combine.

Return half of sole and scallop mixture to work bowl. With machine running, add 3 egg whites one at a time. Add half each of lemon juice, salt, and pepper. With machine running, add half of cream and process until mousse makes soft peaks, no more. When pressed with a finger, surface of mousse should leave indentation mark.

Put mousse mixture in another large bowl and repeat with remaining seafood puree. Combine all seafood mousse in bowl and correct seasoning. Keep chilled.

Put spinach and parsley leaves into work bowl and process until finely chopped. Add 1 cup seafood mousse and process to combine. While machine is running, sprinkle in gelatin. You may season mousse further with lemon juice, and salt and pepper to taste. Set aside 2 tablespoons green mousse.

Chop up a few pieces of pimento and stir into rest of green mousse. Chop olives finely and mix with reserved 2 tablespoons green mousse. Keep all mousse chilled.

Butter a 1 1/2-quart loaf pan. Cut waxed or parchment paper to fit bottom and sides of pan, with enough overhang to cover top. Butter both sides of paper. Spoon about 2/3 white seafood mousse into pan. Using wet spatula, make a V-shaped trough down center of mousse to form upside-down pine tree. Put a row of pimento in apex of V to form star on top. Fill V with green mousse; be careful not to smudge edges.

To form trunk, make a row down center of pan with black olive mousse mixture. Then carefully cover with remaining white mousse, smoothing top with spatula.

Cover with overhanging paper, then tightly cover with aluminum foil. Set mousse in larger pan and place on oven rack. Pour boiling water in larger pan to come halfway up sides of mousse. Bake in preheated 350-degree F. oven 1 hour, or until a knife inserted in center comes out clean (instant meat thermometer should register 140 degrees F.).

Remove loaf pan from water and let mousse sit for at least 20 minutes. Take off foil and paper on top, then run knife around sides and invert. Carefully peel off paper. Cut loaf into thin slices. To avoid smearing tree pattern, remember to wipe off knife blade after each slice.

Serve hot with beurre blanc or hollandaise, or serve cold with Green Mayonnaise, recipe below. Green Mayonnaise 4 egg yolks 2 teaspoons mustard, or to taste 2 tablespoons lemon juice, or to taste 2-3 cups vegetable oil, to desired consistency 6-8 fresh spinach leaves, trimmed 1/2 cup loose parsley leaves 2 shallots, chopped Salt and white pepper to taste

Put egg yolks in processor with mustard and half of lemon juice. Briefly turn on machine to combine them. With motor running, pour in oil in slow, steady stream until it reaches desired consistency.

When mayonnaise is thick and thoroughly emulsified, add spinach, parsley, and shallots, and process until finely chopped. Season to taste with remaining lemon juice and salt and pepper.