Cut the tops from the peppers, remove the seeds and membranes, and stand the shells in a 6-quart crockpot.
Brown the beef and onion in a skillet for 9 minutes until the beef reaches 160°F, drain the fat, and stir in the garlic.
Mix the beef with the rice, 1 cup tomato sauce, cheddar, salt, oregano and pepper.
Fill the peppers, spoon the remaining tomato sauce over the tops, and cook covered on low for 4 1/2 to 5 hours until the peppers are tender.
Rest for 10 minutes and lift each pepper with a wide spoon.