Heat oil on medium heat in an oven-safe skillet. Add tomato, onion, and mushroom. Saute until onions are soft, add spinach and salt and continue cooking until wilted.
In a small bowl, whisk together egg whites and milk. Pour egg mix overcooked vegetables. As the egg cooks, use a rubber spatula and gently lift the edge of the cooked egg to allow uncooked egg underneath. When egg is 75% cooked, sprinkle with feta cheese. Put in oven and bake until egg is completely cooked and feta begins to brown - about 5 to 10 minutes. Allow to set for 5 minutes before serving. Sprinkle with oregano and serve.