In the skillet, sauté the sausage over medium heat until browned, about 10 minutes. Drain the sausage on a paper towel, then add the chopped onion and continue to cook over medium heat until the onion is slightly soft, about 5 minutes.
Add the drained spinach to the skillet and stir into the sausage mixture. Cook for about 3 minutes, until the spinach is warm. Add the chopped tomatoes and cook for another 2 to 3 minutes.
Pour the meat mixture into the prepared casserole dish, then add the liquid egg whites and top with the grated cheese.
Bake in the preheated oven until the cheese is browned and the egg whites stop bubbling, about 1 hour. Remove from the oven and allow to cool for about 10 to 15 minutes, then serve.
Try these variations
Other things to add: crushed red pepper, diced bell peppers, chili powder.