QUICHE LORRAINE is the best known of all quiches. It is traditionally made in a deep crust, and one slice can serve as a light meal. I prefer it made with a light and delicate custard in a shallower tart shell and served as a first course. Recipes for this classic quiche vary in a number of ways. Half an onion can be chopped, sautéed, and added to the filling; crumbled bacon can be used instead of ham; and the custard can be made with more eggs and heavy cream.