Preheat oven to 350 degrees F. and spray a 9″ pie pan with non-stick spray.
In a small skillet, cook the sausage, add the diced onions when the sausage is almost done, cook until translucent. Once cooked, set aside. Add the flour and whisk until all the lumps are gone.
In a large bowl, whisk the eggs and milk together. Stir in the butter. Add the spinach, oregano, parsley, rosemary, cheese, and sausage mixture. Mix until well combined.
Pour mixture into the prepared baking dish, top with the sliced tomatoes and mushrooms.
Bake for 40-50 minutes or until golden brown and a toothpick inserted in the center comes out clean. Remove from the oven and serve immediately.