Preheat your oven to 375°F (190°C).
Heat the olive oil in an 8-inch oven-safe non-stick skillet over medium heat.
Add the diced red bell pepper to the skillet and sauté for 3-4 minutes until slightly softened.
Add the baby spinach to the skillet and cook just until wilted, then spread the vegetables evenly across the bottom.
In a medium bowl, whisk together the large eggs, egg whites, Greek yogurt, sea salt, and black pepper until smooth and well combined.
Pour the egg mixture over the vegetables in the skillet, ensuring they are evenly covered.
Sprinkle the crumbled feta cheese evenly over the top of the egg mixture.
Transfer the skillet to the preheated oven and bake for 15-18 minutes, or until the center is set and the edges are lightly golden.
Remove from the oven, garnish with freshly chopped chives, and slice into wedges to serve.