Add the eggs to a large mixing bowl and whisk.
10 eggs
Whisk in the milk, salt, pepper and cayenne if using.
3 cups whole milk, 1 teaspoon salt, ½ teaspoon ground black pepper, ⅛ teaspoon cayenne pepper
Add the stuffing bread cubes to the eggs mix and stir to moisten them evenly.
8 cups unseasoned stuffing bread cubes
Let the mix sit for 20 minutes until the bread soaks up the milk.
Preheat the oven to 375ºF and spray a 9x13 baking pan with cooking spray. Set the pan aside.
While the bread is soaking, add the butter, onions, pepper and mushrooms to a large skillet and heat over medium high heat. Cook for about 10 minutes to soften the veggies, stirring occasionally.
¼ cup butter, 1 medium onion, 1 green bell pepper, 1 red bell pepper, 1 cup sliced mushrooms
Stir in the broccoli and cook for another 5 minutes to get the excess water out of the veggie.
10 ounces frozen broccoli florets
Remove the skillet from the heat and stir in the sliced tomatoes. Set aside.
1 cup halved cherry tomatoes
Stir in the veggies and ½ cup of cheddar cheese and ½ cup of gruyere cheese into the bread and egg mix.
1 cup shredded sharp cheddar cheese, 1 cup shredded gruyere cheese
Pour the mix into the 9x13 pan and spread evenly across the bottom of the dish.
Sprinkle the remaining cheese over the top of the casserole.
Bake for 40 minutes. The top will be golden browned and puffed up. The eggs will be set and firm to the touch.
Serve the casserole while hot.