A gas grill can, most definitely, be used to make many of the same items you would cook in an oven -- and some charcoal grill enthusiasts put down gas grills as essentially nothing more than an outdoor oven. While this point is certainly debatable, what is true is that setting up your grill to act as a makeshift oven to cook frozen, family-style, dishes like lasagna is a relatively simple process.
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Use Indirect Heat
With a dish like frozen lasagna, it is best to use indirect heat. Direct heat may cause the bottom layer or layers of pasta to burn before the middle and top of the lasagna are reheated or cooked through. Preheat the grill on high for 15 minutes and then, once heated, scrape down the grilling grate with a grill brush. After it's heated, turn one-half of the grill’s burners to medium-low and the other one-half of the grill set to off. Allow the grill to cool for five to 10 minutes.
Prepare the Lasagna
To ready the lasagna for the grill, move it into a heavy-duty, grill-safe pan, or for even more convenience and less cleanup, place the lasagna in a disposable aluminum grill pan. Cover the pan, with the lasagna inside, with aluminum foil. Poke several holes in the aluminum foil to allow steam to escape from the lasagna as it cooks.
Cook the Lasagna
Place the lasagna on the cool side of the grill and close the cover. While the lasagna cooks, turn the pan every five minutes or so, rotate lasagna to ensure even cooking. Cook the lasagna until it's cooked through. Cooking times will vary greatly depending on the size, makeup and thickness of the lasagna. You can check for doneness by inserting a clean, instant-read meat thermometer into the center of the lasagna. The lasagna is fully cooked through and ready to eat once it reaches 165 degrees Fahrenheit.
If you plan to regularly use your gas grill as an extra oven, a quality grill thermometer is a wise, inexpensive item you can use to monitor the heat level in your grill and get more accurate cooking temperatures. This is true even if your grill already has a thermometer. Many built-in grill thermometers are inaccurate, even on high-end grills, and they typically only take the temperature in the center of the grill. For indirect cooking, where only one-half of the grill is generating heat, this would not give you the temperature of the cooking area.