Preheat oven to 400°F (200°C). Slice the tops off the peppers and remove seeds and membranes. If they wobble, shave a thin layer off the bottoms to level them (don't puncture). Brush with 1 tbsp olive oil, sprinkle with salt, and place in a baking dish. Roast for 10–12 minutes to soften slightly.
If you haven't already, cook brown rice according to package directions. Day-old rice works perfectly (FYI, better texture).
Heat remaining 1 tbsp olive oil in a large skillet over medium-high. Add the ground beef, season with salt and pepper, and cook until browned, breaking it up, about 5–7 minutes. Drain excess fat if needed.
Add onion and cook 3–4 minutes until softened. Stir in garlic, smoked paprika, oregano, cumin, and red pepper flakes; cook 30 seconds until fragrant.
Stir in tomato paste and cook 1 minute. Add crushed tomatoes and 1/2 cup broth. Simmer 3–4 minutes until slightly thickened. Taste and adjust salt/pepper.
Add the cooked brown rice (and any optional corn/beans/zucchini). Stir until cohesive. You want a moist, spoonable mixture—not soupy. If dry, splash in a bit more broth.
Spoon the filling generously into each par-roasted pepper, mounding slightly. Pour about 1/3 cup broth into the baking dish around the peppers to create steam.
Cover loosely with foil and bake 15–20 minutes. Remove foil, top with cheese, and bake another 8–10 minutes until melty and lightly golden.
Let peppers rest 5 minutes. Garnish with chopped parsley or cilantro. Serve hot—with a squeeze of lemon if you like contrast.