In a large bowl, combine the ground beef, rice, salt, pepper, and grated onion. Toss gently with your hands or a spatula to combine.
Pour the condensed tomato soup and water in a 13x9-inch baking dish. Stir gently to combine.
Form the ground beef mixture into 16 meatballs (give or take), approximately 1½ inches in diameter.
Place the meatballs into the tomato soup mixture.
Cover and bake for 45–50 minutes, or until the meatballs are cooked through and the rice has popped out.
Serve over more rice, or with mashed potatoes or egg noodles.
Notes
You can use 80% or 90% lean ground beef if you prefer. Make sure the rice is long grain white rice, not Minute Rice or any short grain rices like arborio. They won't work as well. Finely chopped onion works well in place of grated onion. Leftovers will keep for 4–5 days in the refrigerator. Reheat in the microwave, in the oven, or on the stovetop.A reader named Rhea says, "For porcupine meatballs we change things up sometimes by using mushroom soup instead of tomato soup! Both are delicious but totally different!"