If using, fry bacon in a frying pan to desired crispiness, Remove and let drain on a paper towel and crumble when cool.
In a large skillet, heat butter or oil over medium heat. Saute onions for 3 minutes or until tender. Add chopped spinach and saute 3 minutes longer. Remove from the heat and set aside.
In a large bowl, beat the eggs, ricotta or feta, milk and bacon crumbles(if using) salt and pepper until smooth. Stir in the spinach mixture. Pour into a greased 9 by 13 baking pan. Sprinkle with Parmesan or cheddar cheese.
Bake, uncovered, at 350° for 25-30 minutes or until a knife inserted near the center comes out clean. Let sit for 8-10 minutes and cut into squares.
Recipe by Three Hungry Boys at https://www.threehungryboys.com/2020/04/01/spinach-and-ricotta-egg-bake/