In a medium bowl, whisk together the egg whites, whole eggs, Greek yogurt, sea salt, and black pepper until the mixture is smooth and slightly frothy.
Heat the olive oil in a non-stick skillet over medium heat.
Add the sliced mushrooms and diced red bell pepper to the skillet, sautéing for 3-4 minutes until the vegetables are tender and the mushrooms are golden.
Stir in the baby spinach and cook just until wilted, about 1 minute.
Pour the egg mixture over the sautéed vegetables, tilting the pan to ensure even coverage.
Let the eggs cook undisturbed for 2-3 minutes, then gently lift the edges with a spatula to allow uncooked egg to flow underneath.
Once the top is mostly set, sprinkle the feta cheese over one half of the omelette.
Carefully fold the omelette in half and cook for another 30-60 seconds to melt the cheese.
Slide the omelette onto a plate and garnish with freshly chopped chives before serving.