Melt butter in a large sauté pan over medium heat then add the diced onion and sliced mushrooms. Cook, stirring occasionally, until the mushrooms are browned and sizzling, around 4-5 minutes. Add the spinach and cook until wilted.
Add the vegetable mixture to a greased 9 by 13 baking dish and top with 1 cup of the cheese.
Whisk the eggs, salt, pepper, and half and half together in a large bowl then pour over the vegetable mixture. Sprinkle the remaining ½ cup of cheese over top.
Bake for around 30-35 minutes until set. Let cool for 10 minutes before serving.
Notes
Cooked casserole can be frozen for up to 3 months. To reheat, thaw overnight in the refrigerator then bake in a 350ºF oven until warmed through.