Heat the oven to 375°F. Lightly grease 8 cups of a standard muffin pan.
In a large bowl, combine the ground beef, cooked rice, diced bell pepper, onion, egg, breadcrumbs, garlic powder, salt, black pepper, and ¼ cup of the tomato sauce. Mix gently until combined. Avoid overmixing so the muffins stay tender.
Divide the mixture evenly among the 8 prepared muffin cups. Press it in lightly, leaving the tops rounded rather than packing the mixture tightly.
Stir the ketchup into the remaining ½ cup tomato sauce. Spoon a little over each meatloaf muffin, spreading it across the top.
Bake until the muffins are cooked through and the centers reach 160°F, about 25 to 30 minutes. Let them rest in the pan for 5 minutes before carefully removing and serving.