For an extra layer of flavor, try sautéing the onions, mushrooms, and garlic in a little olive oil before adding them to the egg mixture. Feel free to substitute other vegetables based on your preferences and what you have on hand. Bell peppers, zucchini, or spinach would all be delicious additions. If you don't have bacon bits, you can use crumbled cooked bacon or diced ham. This casserole can be assembled ahead of time and refrigerated overnight. Just add a few extra minutes to the baking time.