1. Preheat oven 410F (210C).
2. Preparation: slice onion and mushrooms, crush garlic cloves, roughly chop basil leaves.
3. In a large bowl beat the eggs, cheese, salt and pepper, set aside.
4. In a large skillet heat olive oil over medium heat, add sliced onion and sauté for 4-5 minutes, stirring occasionally until translucent. Add crushed garlic, sliced mushrooms and cook for 6-7 minutes, stirring occasionally until softened, add more olive oil if needed. Add baby spinach and cook 2-3 minutes more, stir occasionally until wilted, season with salt. Add chopped basil and stir.
5. Pour egg-cheese mixture into the skillet, cook for 4-5 minutes. Grate some Parmesan cheese on top (optional) and bake for 15 minutes or until fully cooked and golden brown on top.