Bring a pot of lightly salted water to a boil. Reduce heat to medium-high to maintain a rolling simmer. Cook peppers until starting to get soft, about 5 minutes. Transfer to a paper towel-lined plate to drain.
Heat a nonstick skillet over medium heat. Add ground beef, sausage, and onion. Cook and stir, breaking up the beef, until onion is soft and meat has lost all its pink color, 5 to 6 minutes. Transfer to a bowl with a slotted spoon.
Mix spaghetti sauce, 1/2 cup mozzarella cheese, ricotta cheese, and cooked rice into the beef mixture. Divide among 4 bell peppers and transfer peppers to a baking dish. Sprinkle remaining mozzarella cheese on top of the peppers.
Bake in the preheated oven until cheese melts and begins to brown, 25 to 30 minutes.