Whisk together the eggs, egg whites, Greek yogurt, sea salt, black pepper, and garlic powder in a medium bowl until the mixture is smooth and well-combined.
Heat the avocado oil in a large non-stick skillet over medium heat.
Add the diced red onion to the skillet and sauté for 3-4 minutes until the onion is translucent and fragrant.
Add the fresh spinach to the skillet and cook for 1-2 minutes, stirring constantly until the leaves are just wilted.
Reduce the heat to medium-low and pour the egg mixture into the skillet over the vegetables.
Allow the eggs to sit for 30 seconds, then gently fold in the crumbled feta cheese.
Use a spatula to slowly push the eggs across the pan, creating soft, creamy curds until they are just set but still moist.
Remove from heat immediately and serve warm for the most velvety texture.