Preheat your oven to 375°F (190°C) and lightly grease a small oven-safe baking dish with a touch of olive oil.
In a medium skillet over medium heat, add the olive oil and sauté the diced yellow onion until translucent, about 3-4 minutes.
Add the ground turkey sausage to the skillet, breaking it apart with a wooden spoon, and cook until browned and fully cooked through.
Stir in the minced garlic and fresh spinach, cooking just until the spinach is wilted and the moisture has evaporated.
In a mixing bowl, whisk together the large eggs, egg whites, sea salt, black pepper, and red pepper flakes until well combined.
Transfer the sausage and spinach mixture into the prepared baking dish, spreading it out into an even layer.
Pour the egg mixture over the sausage and spinach, ensuring everything is submerged and evenly distributed.
Bake for 20-25 minutes, or until the eggs are set in the center and the top is lightly golden brown.
Remove from the oven and let it rest for 5 minutes before slicing and serving warm.