Preheat oven to 350. Grease 13X9 baking dish. In a medium bowl, beat eggs.
In a skillet, heat oil. Add spinach. Cook until spinach wilts. Add spinach to prepared dish. Cook mushrooms in same pan until soft. Add cooked mushrooms to dish.
Pour eggs over spinach and mushrooms in dish. Bake at 350 for 45 minutes or until eggs are set. Sprinkle with feta.