When it comes to these two dishes, the key difference is not so much the ingredients as it is simply the preparation and execution. All Recipes notes that both the frittata and the breakfast casserole showcase an egg-based custard mixed with fillings that usually consist of veggies, meat, and cheeses. In both cases, what you put inside your dish is a dealer’s choice kind of situation.
A frittata’s cooking process is two-part, whether you do it on the stovetop flipped or finished in the oven, so it’s a more involved process. A breakfast casserole can be layered and prepared well in advance, and its cooking process occurs all in one go (per The Kitchn) so you can relax the entire time it’s in the oven. Whether you try making a frittata or a breakfast casserole for you and your family or friends, or if you order one off a breakfast menu, you can be sure you’re going to have a hearty, tasty meal combination of all of your favorite breakfast ingredients.