Use as much onion as you like, but to keep the ground beef from getting too watery, one medium onion is enough. Peel and chop the onion, peel a couple of garlic cloves, trim the bell pepper, and chop the cilantro. Blend everything together and add a couple of generous tablespoons to the ground beef, mixing well to combine.

Add salt and pepper to taste. Place a tablespoon of the seasoned ground beef in the palm of your hand, shape it into a small bowl, and add a reasonable amount of your preferred cheese (either shredded or cut into 3/4-inch cubes). Bring the edges of the meat up to enclose the cheese and gently press to seal. Repeat with the remaining mixture.

Once all the meatballs are formed, prepare the sauce. Mince 2 garlic cloves and chop another onion into small cubes or thin slices. Sauté until softened, then add the tomato sauce. Cook with a little water for at least 1 hour to reduce the acidity of the tomatoes. Make sure it doesn't dry out too much, especially if you plan to serve the dish with pasta.