Preheat your oven to 375°F (190°C) and lightly grease a small oven-safe baking dish or 6-inch ramekin with a touch of olive oil.
Heat the olive oil in a skillet over medium heat and sauté the diced yellow onion and red bell pepper until softened, roughly 4 minutes.
Add the ground turkey to the skillet, breaking it into small crumbles with a spatula, and cook until fully browned and no longer pink.
Stir in the fresh spinach and cook just until wilted, then remove the skillet from the heat to cool slightly.
In a medium mixing bowl, whisk together the large egg, egg whites, non-fat Greek yogurt, nutritional yeast, sea salt, black pepper, and garlic powder until the mixture is smooth and aerated.
Fold the sautéed turkey and vegetable mixture into the egg batter until the ingredients are evenly distributed.
Pour the batter into the prepared baking dish and bake for 25 to 30 minutes, or until the center is firm and the edges are beautifully golden brown.
Remove from the oven and let the protein cake rest for 5 minutes before slicing into wedges to serve.