If that nagging feeling that you've forgotten something turns out to be about a 3-pound frozen lasagna, you've got a little math to do. When it's properly thawed, heating a large baked or unbaked lasagna is fairly straightforward. But if you've neglected to move your prize pasta dish from the freezer to the refrigerator, you'll have to factor in extra cooking time.
"For Best Results..."
There's a reason your cookbook strongly recommends thawing frozen lasagnas before cooking them. Because of its large size, using the "thaw" setting on your microwave is impossible. For obvious reasons, it takes longer to bake frozen food than refrigerated or room-temperature food. The larger the lasagna, the longer it takes to thaw. In addition, if the lasagna is put into the oven while solidly frozen, you risk that it will emerge with either a cold center or an overcooked perimeter. Whenever possible, set the lasagna in the refrigerator to thaw overnight.
Unbaked and Frozen
Normally, you cook frozen but uncooked lasagna by thawing it in the refrigerator overnight and baking it at the temperature and time specified in the recipe -- generally about an hour for large lasagnas. If you're skipping the thawing stage, however, lower the temperature specified in the recipe by 25 degrees and add 30 minutes to the cooking time. Before serving, check the center with a knife to make sure there aren't any frozen spots.
Baked and Frozen
To take a large homemade baked lasagna from freezer to oven, preheat the oven to the temperature specified in the baking recipe. Estimate that it will take twice as long to cook the frozen lasagna as it did when baking it, at the same temperature called for in the baking recipe. Begin checking before that estimated time, though, so that you can remove it once the sauce is bubbly and the center feels cooked through.
Store-bought frozen lasagnas are made to go directly from freezer to oven. The times for heating large "family size" lasagnas vary depending on the brand, but they will be spelled out plainly on the packaging. In general, the largest sizes take more than an hour. The lasagna goes into a preheated oven on a baking sheet, usually with its plastic cover still on but vented with a knife. After 45 minutes or the time mentioned on the packaging, remove the covering and cook for at least another 25 minutes.