- Preheat oven to 375 degrees. Spray a casserole dish with cooking spray.
- Heat a large skillet with the butter and olive oil in it. Add the mushrooms and cook until soft. Add in the spinach and cook until wilted.
- Spoon the vegetables into the casserole dish.
- Slice the sausage into thin slices. Add the sausage to the same skillet and cook for about 5 minutes. Put the sausage in the casserole dish on top of the vegetables.
- Whisk the eggs, heavy cream, red pepper flakes, salt and pepper together. Combine the ingredients well and then stir in the cheddar cheese.
- Pour the egg mixture into the casserole dish. Cover the dish and bake for 15 minutes. After 15 minutes remove from oven and uncover the dish and top with the mozzarella cheese. Bake for 15 minutes more uncovered. The cheese should be lightly browned and bubbly.