Preheat oven to 400 degrees F. and prepare a 9 x 13 dish with a natural cooking spray.In a large skillet, cook chopped bacon and onions until bacon is of desired doneness. Drain excess grease from pan, as needed.Meanwhile, prepare stuffing mix according to package instructions and set aside. (I did not use the butter)Add the prepared stuffing and the spinach to the skillet with the bacon and mix together until thoroughly combined. Transfer to a 9 x 13 baking dish and sprinkle with Monterey Jack cheese.In a small bowl, whisk together the half-and-half and/or milk and eggs. Pour evenly over stuffing/bacon/spinach mixture.Bake at 400 degrees F. for about 35 minutes. Sprinkle with Parmesan cheese and bake an additional 5 minutes and until set.Allow casserole to cool for about 5 minutes before serving.