Generally, lasagna is cooked covered for the majority of the cooking time, and then uncovered for the last 10-15 minutes to allow the cheese on top to brown and become bubbly. This helps to ensure that the lasagna is cooked evenly and that the cheese on top is melted and golden brown. However, the exact cooking time and method can vary depending on the recipe, so it's always best to follow the instructions provided in the specific recipe you are using.