Pour tomato sauce into a heat proof bowl and add diced onion. Heat the onion/tomato sauce mixture in the microwave for about 5 minutes to soften onions. Set aside to cool.
Cut the tops off of bell peppers. Remove stems and seeds and dice the tops of the bell peppers. Pour 1/3 of a cup of water into the bottom of the crock pot, add a cooking rack or foil balls to the bottom of crock pot and place bell peppers on top.
In a large mixing bowl, combine one pound of uncooked ground beef, cooled onion/tomato sauce mixture, diced bell peppers, 1 cup of shredded colby jack cheese, 3/4 cup of cooked mexican rice, 1 1/2 teaspoons of kosher salt and 1/2 teaspoon of pepper.
Fill bell peppers with ground beef mixture and top with remaining cheese. Cook on low for about 6 hours or high for 3 hours. You can test mixture with meat thermometer for doneness. It should be cooked to 160 degrees.
You must used pre-cooked rice. I used leftover mexican rice, but you can also use cooked white rice. Just bump up your seasoning so that the mixture is not too bland. You can add chili powder, garlic powder, cumin, chipotle in adobo. Whatever sounds good to you!