1. Preheat a skillet to medium-high heat with a little avocado oil, tallow, or butter. Add the diced onion, bell pepper, and mushrooms and cook 3-4 minutes. Add the ground sausage and cook until no longer pink, about 6-8 minutes, breaking it up as it cooks. Add the chopped spinach and cook for a minute or two until wilted.
2. Grease the bottom of a 9×13 baking dish and layer the cooked veggies and sausage mixture on the bottom. Sprinkle the shredded parmesan cheese over top.
3. In a large bowl, whisk together the egg whites with the milk, salt, and granulated garlic. Adjust the salt as needed. My ground sausage already had salt so 1/2 tsp was sufficient for my casserole.
4. Carefully pour the egg white mixture into the baking dish over top of the base mixture.
5. Bake at 350ºF for 45-55 minutes, or until the center is completely set.
6. Let it rest for 10-15 minutes before slicing.

Notes:

• I only had 24 oz of egg whites on hand, but you can absolutely use an entire 32-oz container of egg whites if you have store-bought egg whites.
• You can use sausage links instead of ground sausage and just chop it up.
• Substitute or add other veggies, such as sun-dried tomatoes, leeks, fennel, etc.
• Substitute any other cheese in place of the parmesan, such as cheddar, mozzarella, gouda, etc.
• To make the recipe dairy-free: Omit the cheese and use a dairy-free milk.