Preheat the oven to 190°c/375°F/Gas 5. 

Fry the onion and garlic in a frying pan (with an ovenproof handle) until translucent and soft. Add the mushroom and let it cook with the rest of the ingredients until they have shrunk and softened up.

Add the spinach to the mixture and cook until they are wilted.

Whisk together the egg and egg whites in a separate bowl and pour over the mixture in the skillet.

Place the frying pan in the oven and bake for 12-15 minutes, or until browned and eggs are completely cooked.