In order to produce a great gratin, you must let the grated or shredded cheese fall slightly and evenly on the surface. If you’re using a lot of cheese, let it build up in a fluffy layer so the oven’s heat penetrates the granules, baking them into a crisp and generously colored crust. Resist the temptation to pat or press the cheese down, or it will turn into a gummy slab when it bakes.
Often cooks remove their lasagna or manicotti when they see the first streaks of gold, afraid that it will burn in another minute. When the cheese has been applied in a light touch, it will darken gradually for a long time without burning. Not only will longer baking produce a more spectacular looking dish, but the deeper caramelization creates more flavors too. For me, a great gratin must be dark and deeply colored all over the top.