BON APPETIT!Two musical monologues with music by Lee Hoiby, words by Ruth Draper and Julia Child. Directed by Carey Perloff. Starring Jean Stapleton. At the CSC Theater through Nov. 3. 'BON APPETIT!" offers a double portion treat for gourmet theatergoers. In the first show of its silver anniversary season, the CSC Theater features the work of three remarkable women: author-monologuist Ruth Draper, French chef extraordinaire Julia Child, and multi-talented singing actress Jean Stapleton. Lee Hoiby has composed choice musical settings for the words of Draper and Child. The solo entertainment has been deftly directed by Carey Perloff. The rest is up to Miss Stapleton, who responds with he r own blithe spirit and versatility to the demands of the occasion. The 90-minute diversion opens with Draper's memorable 1926 monodrama, "The Italian Lesson." The not unkindly satire records a day in the life of a Manhattan society matron as she grapples with Dante's "Divine Comedy" while conducting domestic and social affairs. Among many things, she instructs her cook on the dinner (beginning with "a clear soup with something amusing in it"); welcomes the family's new puppy (they'll call him Dante); advises her secretary ("Before you do anything, try to get some men fo r the opera Monday."); and deals with her various offspring. Composer Lee Hoiby's piano runs to simulate the ringing telephone is one of a number of pleasant musical touches, all nicely handled by musical director and accompanist Todd Sisley. Philosophically speaking, "The Italian Lesson" is a metaphor. Draper's unnamed heroine begins by struggling with Dante's "In the middle of the journey of our life...." and ends by instructing the secretary to order a new book, "Our Inner Life" ("I don't know who wrote it but I have to discuss it at the Book Club"). The short second half of "Bon Appetit!" recalls a 1970 Julia Child broadcast in which the French chef prepared Gateau au Chocolat l'Eminence Brune. Stapleton's Child incorporates her subject's inimitable treble, stance, and common-sense gastronomic guidance. After announcing that "today we are going to make a chocolate cake," she adds instructively: "When you do a cake, you really have to have a battle plan." Whereupon Child becomes a singing French chef for the occasion. Stapleton enlivens the musical p ortrait with virtuosity as actress and singer. Donald Eastman has fashioned functionally simple settings for the two musicals sketches and Mary Louise Geiger has lighted them becomingly. Stapleton's lush green house gown and a festoon of pearls for "The Italian Lesson" were designed by Rita Briggs; Jean Putch created the simple garb (not forgetting the indispensable dish cloth) for Child.