For an extra layer of flavor, sauté the mushrooms with a clove of minced garlic and a tablespoon of butter before adding them to the casserole. Feel free to substitute other cheeses, such as cheddar, Gruyere, or mozzarella, for the Monterey Jack. Add a pinch of nutmeg to the egg mixture for a warm, comforting note. If you don't have frozen spinach, use fresh spinach that has been lightly sautéed and drained. This casserole can be easily doubled or tripled for larger gatherings.