Preheat your oven to 400 F.
Coat an 8 inch oven safe skillet with olive oil.
Place in on your stovetop and turn burner to medium heat.
Add onion and garlic and sautee for 5 minutes. Add the oregano and spinach and continue sauteeing until the spinach has wilted.
In a mixing bowl, add the eggs, milk, and salt (if using). Mix well.
Pour the egg mixture over the onion and spinach mixture in skillet.
Sprinkle olives and feta cheese on top. Top with sliced tomatoes.
Bake for 20-25 minutes until eggs are fully cooked through.