Preheat your oven to 375°F (190°C) and lightly grease a small oven-safe skillet or baking dish with a touch of oil.
Finely dice the red bell pepper and yellow onion, then slice the lean chicken sausage into thin, uniform rounds.
Heat the extra virgin olive oil in a skillet over medium heat, then sauté the sausage, peppers, and onions until the vegetables are tender and the sausage is lightly browned.
Add the baby spinach to the skillet and stir for about 60 seconds until it is just wilted, then remove the pan from the heat to cool slightly.
In a medium mixing bowl, vigorously whisk together the large eggs, egg whites, sea salt, black pepper, and garlic powder until the mixture is well-combined and frothy.
Pour the egg mixture directly over the sautéed ingredients in the skillet, ensuring the sausage and veggies are evenly distributed.
Sprinkle the crumbled feta cheese across the top for a tangy finish and place the dish on the center rack of the oven.
Bake for 18 to 22 minutes until the eggs are fully set in the center and the top has achieved a beautiful golden-brown hue.
Let the frittata rest for 5 minutes before slicing into portions, making it the perfect high-protein start to your morning.