1. Preheat the oven to 350 degrees F and lightly coat a 13 X 9 baking dish with cooking spray; set aside.
2. Cook bacon in a large skillet until crispy; reserve 1 tablespoon of drippings, and drain the remainder; pat bacon with a paper towel and set aside. Crumble when cool.
3. In the same skillet, cook onions in reserved bacon drippings until soft and translucent, approximately 5 minutes; remove and set aside.
4. In the same skillet, cook spinach for 2 minutes, or until wilted.
5. In a large bowl, mix eggs with salt, pepper, and cheese; add spinach, onions, and bacon and stir until well combined.
6. Pour into casserole pan and bake for 30-40 minutes or until eggs are set. Cool for 5 minutes before serving.

Make ahead:

You can prepare this recipe a day before serving! Complete all steps above (but do not bake) and cover the casserole dish tightly to refrigerate. Remove casserole from refrigerator 30 minutes before baking to allow to come to room temperature. Follow baking instructions as stated above.