Preheat the oven to 175C and prepare a baking dish or your cast iron pan.
Crumble the ricotta cheese into the egg mixture and season to taste.
Melt the butter in a pan on medium heat and saute the onions for 3 minutes.
Add the sliced mushrooms and saute for another 3-5 minutes.
Add the spinach and saute for 2-3 minutes.
Place the onions, mushrooms and spinach in the bottom of the pan and pour the egg mixture over.
Place the pan, uncovered in the oven and bake for 15 minutes. If you are using a cast iron remember it will stay hotter for longer and the eggs may become overcooked. If you are using a regular skillet you may need to cook the eggs for 20 minutes instead of 15 minutes.