Preheat oven to 375°F (190°C).

In a bowl, whisk together egg whites, whole eggs, milk, salt, pepper, and oregano.

Heat olive oil in an oven-safe skillet over medium heat.

Sauté onion until translucent, about 3-4 minutes.

Add spinach and cook until wilted, about 2 minutes.

Pour egg mixture over the vegetables and sprinkle feta on top.

Cook on stovetop for 3-4 minutes until edges start to set.

Transfer skillet to preheated oven and bake for 10-12 minutes until fully set and lightly golden.

Let cool for a few minutes before slicing and serving.