Preheat your oven to 375°F (190°C) and lightly grease a small oven-safe baking dish with a touch of olive oil.
Heat 1 teaspoon of olive oil in a skillet over medium-high heat. Add the ground turkey sausage, breaking it up with a spatula, and cook until browned and no longer pink.
Add the diced red bell pepper and baby spinach to the skillet. Sauté for 2-3 minutes until the spinach is wilted and the peppers are slightly softened.
In a medium mixing bowl, whisk together the 3 large eggs, 0.5 cup of egg whites, sea salt, black pepper, and garlic powder until well combined and slightly frothy.
Spread the cooked sausage and vegetable mixture evenly across the bottom of the prepared baking dish.
Pour the egg mixture over the sausage and vegetables, then sprinkle the crumbled feta cheese evenly across the top.
Bake for 25 to 30 minutes, or until the eggs are completely set in the center and the top is beginning to turn a light golden brown.
Allow the casserole to cool for 5 minutes before slicing and serving warm.