Preheat oven to 350 degrees. Prepare two 9x13 casserole dishes with nonstick cooking spray.
In a large skillet over medium-high heat, melt the butter. Add the onion, bell pepper, and mushrooms. Cook, stirring as needed, until the onion has softened.
Place the spinach in the bottom of the casserole dishes. Top with cooked vegetables.
In a large bowl, beat together the eggs, milk, and seasonings. Pour over the casseroles.
Cover the casseroles with cheese.
Bake for 45 to 60 minutes or until the center is set and the cheese is melted.