Prep the peppers: slice off tops, remove seeds, and trim bottoms so they stand upright.
Warm olive oil in a skillet over medium heat. Soften the onion for 3–4 minutes, then stir in garlic.
Brown the ground beef with the onion mixture, breaking it up as it cooks. Drain any excess fat.
Combine the beef, cooked rice, diced tomatoes, Worcestershire, Italian seasoning, salt, and pepper in a bowl.
Stir together tomato sauce and beef broth in the slow cooker to make a sauce base.
Pack the beef-and-rice filling into the peppers, mounding slightly at the top.
Nestle stuffed peppers upright in the slow cooker over the sauce. Spoon a little sauce over each.
Cover and cook on LOW 4–5 hours or HIGH 2–3 hours, until peppers are tender but hold shape.
Top peppers with mozzarella during the last 10 minutes. Cover to melt the cheese.
Serve peppers with extra sauce from the pot and a sprinkle of parsley.