Lesson by lesson with Lydie Marshall

When Lydie Marshall started teaching cooking at her brownstone in Greenwich Village 10 years ago, she bought directors' chairs for all of her students to sit in. About a half hour into her first class, the chairs were folded up and put away, never to be used again.

Her classes are now total participation, with students working in pairs, and Lydie circulating around the room.

''Having a cooking school is fabulous,'' Lydie says, ''because you get to know people from all walks of life, and at the same time learn to cook and enjoy a good meal together.''

But for those who don't live in New York, Lydie has written a cookbook that imitates the format of her classes - progressing from simple techniques to more difficult, and encompassing an entire menu, with appetizer, main dish, vegetables, and dessert included in each of the 22 lessons.

Cooking with Lydie Marshall (New York: Alfred A. Knopf. $18.95) contains several simple dishes (like a perfect roast chicken with vegetables, perfect mashed potatoes, and a classic caramel custard) and more complex recipes for croissants, poached fish mousse, and puff-pastry dessert tarts.

Although the lessons become more complex as you go through the book, there are always simple dishes to balance the work involved in each menu.

Lydie learned to cook as a child in France by watching her mother, but there were no cookbooks to read for guidance.

Later, when she came to live with her aunt and uncle in the United States, Tatane, her mother's sister, became her principal teacher.

But it was when Lydie was first married and ''Mastering the Art of French Cooking'' by Julia Child, Louisette Bertholle, and Simone Beck was published that Lydie really started cooking.

Here all the recipes and techniques she had in her head were written down and clearly explained. ''It was like my mother talking to me,'' Lydie said.

Lydie's cooking is influenced by her French background, but she has tried to make her recipes sound as American as possible, because she finds that French names intimidate many people when first learning to cook.

She laments that she has never completely lost her French accent, but it seems only to add to her charming manner.

Lydie and her husband have a flat in Paris that they return to each summer, and she exchanges ideas and recipes with her Parisian friends, and complains with dismay about the lack of taste and texture in French bread today.

The appealing blue and white print cover of the book was adapted from one of Pierre Deux's provincial fabrics originally named after her, ''Lydie.''

Here is her version of a classic French dessert, creme caramel. The baking and unmolding are the tricky steps. The custard should cook enough so that it will unmold and not collapse, and it should not overcook; otherwise, the custard will be full of holes. Caramel CuAtard Caramel for the mold: 1/2 cup sugar 4 tablespoons water Custard: 3 cups milk 2/3 cup sugar 1/4 teaspoon vanilla extract 1 1/2 cups eggs (6 to 7 large eggs)

To prepare caramel, in heavy-bottomed saucepan over medium heat, stir sugar and water until sugar is dissolved.

Cook syrup for about 5 to 10 minutes until it colors to an amber shade. Using potholders, tip pan to even out color.

Pour caramel into a 4-cup ring mold and quickly swirl the caramel all around sides and bottom of mold. Reserve mold right side up.

To prepare custard, bring milk to a boil, then quickly whisk in sugar and keep cooking until sugar is dissolved, about 1 minute. Turn off heat. Add vanilla. Cover and cool 15 minutes.

Beat eggs for 1 minute in 3-quart mixing bowl and pour milk over them, whisking constantly. Strain eggs-milk mixture through fine-mesh sieve into bowl, then transfer to prepared caramelized mold.

Preheat oven to 400 degrees F. Place mold in water bath. Pour boiling water in water bath until it reaches halfway up the sides of the ring mold.

Bake on middle shelf of oven for 30 to 35 minutes. If top browns slightly during cooking, cover loosely with foil. To test whether custard is done, plunge knife in center. It should come out clean.

Remove from water bath. Cool at room temperature for 30 minutes, then turn mold upside down on serving dish and unmold. Refrigerate.

This is the quickest apple dessert to make, and is especially good when Jonathan apples are in season. Apple Gratin 4 apples, Jonathan, Cortland, golden delicious, or Granny Smith 1 teaspoon lemon juice 4 tablespoons sweet butter 1/4 cup sugar 1/4 cup heavy cream

Preheat oven to 400 degrees F. Peel, core, and quarter apples. Cut each quarter into 1/3-inch slices and sprinkle lemon juice over them.

Butter generously a 1 1/2-quart baking dish and lay overlapping slices of apples in dish. Sprinkle with sugar and add butter, cut into very thin slices.

Bake for 30 minutes. Turn oven to 500 degrees F. Spread cream over apples and bake 5 minutes more, or until apples are slightly golden on top. Serve hot. Serves 4.