1. Preheat the oven to 350 degrees. Spray a 9-inch cast-iron skillet or glass baking dish with cooking spray. 2. Heat a large nonstick skillet over medium heat. Spray with cooking spray. Cook the mushrooms for five minutes, stirring occassionally, until they start to release their liquid. Stir in the spinach and cook for 1 minute. Remove from heat. 3. Whisk together the eggs and milk in a large bowl. Add the mushroom-spinach mixture, tomatoes, and cheese. 4. Pour the egg mixture into the skillet. Bake for 35 to 45 minutes. Cool for 5 minutes before cutting.