To reheat lasagna in the oven, transfer the lasagna in an ovenproof pan and cover the dish with a lid or a large piece of aluminum foil. Place the pan into a preheated 325-degree Fahrenheit oven to bake until it is heated through.
After 20 minutes, insert the tip of a knife into the center of the lasagna, and allow it to rest there for 10 to 15 seconds. Remove the knife, and feel the tip with a hand or finger. If the tip is hot, the lasagna is heated through and ready to eat. If the tip is not hot, place the lasagna back into the oven, and check in 10-minute intervals until the tip of the knife is hot to the touch. This means the lasagna is heated and ready to serve.