Heat

• The process of baking a cheese lasagna has two objectives: to heat the dish to a temperature sufficient to fully cook all of the ingredients, and to slightly brown the top layer of mozzarella and parmesan cheeses. These multiple objectives require different baking times and require different types of heat. When you place a pan of lasagna in a hot oven, the heat will penetrate the outside layer before working its way into the inner layers. Covering the pan helps to retain the heat and enables the lasagna to cook more evenly.

Moisture

• Lasagna ingredients contain plenty of moisture, from the viscous sauce, to the boiled noodles, to the fresh ricotta cheese. The baking process should heat the dish while retaining most of this appealing moisture. If you use no-boil lasagna noodles, it is especially important to keep the dish moist while it bakes, because these noodles steam as they cook. Covering the pan for most of the baking time seals in the moisture.

Baking Process

• Although most recipes instruct you to cover lasagna for most of the time it bakes, they also tell you to remove the foil for the last five to 10 minutes to let the cheese brown. Cooking the lasagna uncovered for the final part of the baking process adds an appealing crustiness to the top layer of cheese but prevents the overcooking that would occur if you baked the lasagna uncovered for the entire baking time.

Cheese Behavior

• Ricotta is a moist, fresh cheese that doesn't typically brown or dry out, especially when it is protected by layers of pasta. Covering lasagna with foil during the baking process won't appreciably change the way the ricotta cheese cooks, because it is so moist and it is insulated by layers of noodles. The mozzarella and parmesan cheeses that you use to top the lasagna will melt if the dish is covered, but they won't brown until you uncover the pan for the last stretch of cooking time.