Add the avocado oil to a large thick-bottomed pot and heat to medium-high.
Add the diced yellow onion and sauté, stirring occasionally, until the onion turns translucent, about 5 minutes.
Add the ground beef and allow it to brown for a couple of minutes before chopping it into smaller bits and mixing it in with the onion.
Add the remaining ingredients except for the cooked rice and cook at a full boil for 8 to 10 minutes, stirring occasionally, or until the sauce surrounding the meat is nice and thick. Stir in the cooked rice.
Preheat the oven to 400 degrees F.
Cut the tops off of the bell peppers and remove the core.
Fill the bell peppers with the ground beef mixture (there should be enough mixture for 5 or 6 medium-sized or 4 large bell peppers).
Place the bell peppers in a deep casserole dish and cover with foil. Bake for 30 minutes. Remove the peppers from the oven and sprinkle each with desired amount of cheese. Place the peppers back in the oven and bake for another 10 minutes, or until cheese is nice and melted.