Spray the crockpot with cooking spray. Spread a small layer of tomato sauce around the bottom of the pot.
Wash the peppers & slice off the tops with a slight diagonal cut. Stuff each pepper with the prepared stuffing.
Lay the peppers in a single layer on top of the tomato sauce. Add a layer or two of meatballs on the top of the peppers (depending on how many people you're going to feed).
Pour the remaining tomato sauce evenly on top of the meatballs. Cover. Cook on low for 3-4 hours. Sprinkle with freshly grated parmesan, if desired.