Preheat your oven to 375°F (190°C) and lightly grease a small baking dish with avocado oil.
In a non-stick skillet over medium heat, add the avocado oil and the ground turkey sausage, breaking it up with a spatula until browned and cooked through.
Add the diced yellow onion and red bell pepper to the skillet, sautéing for 3-4 minutes until the vegetables are tender and fragrant.
Stir in the fresh baby spinach and cook just until wilted, then remove the skillet from the heat.
In a medium mixing bowl, whisk together the whole eggs, liquid egg whites, sea salt, black pepper, and garlic powder until well combined and slightly frothy.
Transfer the cooked sausage and vegetable mixture into the prepared baking dish, spreading it out into an even layer.
Pour the egg mixture over the sausage and vegetables, ensuring everything is submerged.
Sprinkle the shredded sharp cheddar cheese evenly over the top of the casserole.
Bake for 20-25 minutes, or until the eggs are set in the center and the cheese is bubbly and golden.
Allow the casserole to cool for 5 minutes before slicing and serving.