Heat oven to 375°F. Lightly oil a 9x13-inch baking dish.
Prep peppers: trim tops, remove seeds, and set cut-side up in the dish. Brush with a little oil.
Par-bake peppers for 8–10 minutes to start softening. Keep oven on.
Warm remaining oil in a large skillet over medium heat. Add onion and cook until translucent, 3–4 minutes.
Add ground beef and cook, breaking it up, until browned. Drain excess fat if needed.
Stir in garlic, tomato paste, Italian seasoning, smoked paprika, salt, and pepper. Cook 1 minute until fragrant.
Pour in tomato sauce and Worcestershire. Simmer 2–3 minutes, then fold in the cooked rice and red pepper flakes if using.
Fill peppers with the beef-rice mixture, packing gently. Add broth to the baking dish around the peppers.
Cover tightly with foil and bake 20–25 minutes, until peppers are tender-crisp.
Uncover, top with cheese, and bake 10–12 minutes more until melted and bubbly. Garnish with parsley and serve.