Add the onion, stir to cook until the onions are tender, about 5 minutes. If you are using regular spinach, add it right now with your onions.
Place bacon pieces back into your skillet.
Add crushed red pepper flakes, if using, garlic, salt and pepper and stir. Reduce heat if the garlic starts to turn brown. Stir constantly.
Add baby spinach, if using, and cook until it starts to wilt; less than a minute. Exact cooking time will be determined by the heat and type of spinach you're using.
Remove spinach from skillet or wok and place on serving plate.
Sprinkle immediately with cheese of your choice.
Serve immediately. Enjoy!