Then briefly cook it in butter in a large skillet (9″) over medium heat. This is essential to take the raw onion edge off because the baking time isn’t quite enough.
Pour the veggie egg mixture into a well-buttered pie plate or similar-sized baking dish. If you’re using the optional cheese, you can sprinkle it over the top or mix it in.
Bake the egg casserole until puffed in the center and done.
Serve hot out of the oven, or at room temperature. Let the casserole cool to just warm if you want to portion it out for meal prep.