Place the ground beef, uncooked rice, egg, cilantro in a large mixing bowl, and mix to combine well (I use my hands to knead the mixture, but you can also use a spoon.) Season with salt and pepper, and then divide the mixture into 16 equal portions, rolling each portion into a ball.

Heat the olive oil in a large covered Dutch oven. Sauté the onions for a minute or two over medium heat, until they turn translucent. Add the meatballs, and brown over medium heat for about 7 minutes. Turn the meatballs over, and brown for another 7 minutes. (Turn the meatballs only once so they do not break apart.)

Pour in the entire contents of the can of diced tomatoes, and add the minced garlic. Add salt and pepper, if desired. Cover the pan, and lower to simmer for 30-40 minutes, until the rice appears split, which indicates it is fully cooked. Remove the lid from the pan, and simmer for another 5 minutes, to reduce the sauce.