Hello Greg. That is a wonderful initiative to reduce food waste, and weekly casseroles are a fantastic way to do it.
To answer your question directly: no, you do not strictly need to preheat your oven for leftover casseroles. Starting in a cold oven works completely fine, and in many cases, it is actually quite practical.
Preheating is critical for delicate baked goods like breads, cakes, pastries, or souffles, where an immediate burst of heat is necessary for proper rise and leavening. Casseroles, by contrast, are dense dishes where the primary goal is simply heating the ingredients evenly throughout and melting any cheese or browning the top. A gradual rise in temperature allows the heat to penetrate toward the center of the dish without burning the exterior.
Furthermore, if you assemble your casserole ahead of time and keep it in the refrigerator, placing a cold glass or ceramic baking dish directly into a fully preheated oven creates a risk of thermal shock, which can crack or shatter the dish. Starting in a cold oven lets the dish and the food warm up together safely.
The only adjustment you need to make is to your baking time. Since the oven takes 10 to 15 minutes to reach temperature, you should expect to add roughly 10 to 15 minutes to the total time the dish spends inside. The best way to ensure it is finished is to look for bubbling around the edges and check that the center reaches a safe internal temperature of 165 degrees Fahrenheit (about 74 degrees Celsius).
I hope this explanation helps make your weekly cooking routine even smoother. Best of luck with your dishes!
