Bring water and rice to a boil in a saucepan over high heat. Reduce heat to medium-low, cover, and simmer until the rice is tender, 20 to 25 minutes.
Heat olive oil in a large skillet over medium-high heat. Stir in ground sirloin; cook until it begins to brown, and is no longer pink, about 5 minutes. Drain any excess fat. Stir in 1 can tomato sauce, green onions, garlic salt, kosher salt, and pepper; cook until flavors combine, 5 to 10 minutes.
Mix rice into ground sirloin mixture.
Preheat oven to 350 degrees F (175 degrees C).
Stuff Cubanelle pepper halves with the meat and rice mixture, rounding the tops. Place the stuffed peppers into a 9×13-inch baking dish.
Stir remaining can of tomato sauce, brown sugar, parsley, basil, and oregano together in a bowl. Spoon tomato sauce over peppers in the baking dish.
Bake in the preheated oven until peppers are hot and beginning to brown, about 1 hour. Sprinkle mozzarella cheese on top.