Halve and clean the peppers: Cut each pepper straight down the middle through the stem. Remove the stem, white ribs, and seeds. Rinse and set cut-side down on a towel.
Parboil for exactly 3–4 minutes: Drop the pepper halves into boiling water in batches if needed. Boil 3 minutes for firmer peppers or 4 minutes for softer. Remove with tongs, set cut-side down on a clean towel, and pat dry inside with a paper towel.
Brown the beef: Heat olive oil in a large skillet over medium-high heat. Add ground beef, let it brown without stirring for a minute or two, then break it into crumbles and cook until no pink remains, about 5–6 minutes.
Add onion and garlic: Stir in the diced onion and cook 3–4 minutes until soft and translucent. Add the garlic and cook 30 seconds until fragrant.
Drain excess grease: Tilt the skillet and spoon off extra fat, leaving about a tablespoon for flavor.
Build the sauce: Stir in tomato paste and cook 1 minute. Add tomato sauce, Worcestershire, Italian seasoning, smoked paprika, and black pepper. Let it bubble gently for 5 minutes until thickened.
Add rice and cheese: Turn off the heat. Stir in cold rice, 1 cup of shredded cheddar, and Parmesan. Taste and add salt if needed. Filling should be thick, not soupy.
Stuff the peppers: Place parboiled halves cut-side up in a 9x13 baking dish. Mound filling over the top and sprinkle with remaining ½ cup cheddar.
Bake covered: Pour ⅓ cup broth or water into the bottom of the dish. Cover tightly with foil and bake at 375°F for 25 minutes.
Uncover and finish: Remove foil and bake 10–15 minutes more, until cheese is golden and peppers are tender. Rest 5 minutes before serving.