Preheat your oven to 375°F and lightly grease a small pie dish or 8-inch oven-safe skillet.
Finely dice the yellow onion and red bell pepper, and roughly chop the fresh spinach.
Heat the olive oil in a pan over medium heat and sauté the onions and peppers for 4-5 minutes until they are softened and fragrant.
Add the chopped spinach to the pan and stir for 1 minute until just wilted, then remove the pan from the heat.
In a medium mixing bowl, whisk together the whole eggs, egg whites, Greek yogurt, sea salt, black pepper, and garlic powder until the mixture is smooth and well combined.
Spread the sautéed vegetable mixture evenly across the bottom of your prepared baking dish.
Pour the egg and yogurt mixture over the vegetables, ensuring they are evenly covered.
Sprinkle the shredded sharp cheddar cheese over the top of the egg mixture.
Bake for 25-30 minutes, or until the center of the quiche is set and the cheese on top is bubbly and lightly golden.
Remove from the oven and allow the quiche to rest for 5 minutes to firm up before slicing and serving.