Preheat the oven to 350 degrees F (175 degrees C). Grease a 9x13-inch baking dish.
Cut tops off of tomatoes; set aside tops. Scoop out pulp into a bowl and place hollowed tomatoes into the prepared baking dish.
Bring water to a boil in a saucepan. Remove from heat and stir in instant rice. Cover and let stand until water is absorbed, about 5 minutes.
Heat a large skillet over medium-high heat. Cook and stir beef in the hot skillet until browned and crumbly, 5 to 7 minutes; drain and discard grease. Mix cooked rice, onion, garlic, garlic salt, and black pepper into ground beef; stir in tomato sauce and reserved tomato pulp. Bring to a boil, reduce heat to low, and simmer until thickened, 10 minutes.
Fill each hollowed tomato with ground beef mixture. Cover each filled tomato with a tomato top. Pour tomato juice over filled tomatoes for extra juiciness.
Bake in the preheated oven until tomatoes are tender and filling is hot, about 20 minutes.