Preheat oven to 190°C (375°F). Slice the top off each pepper, remove seeds and membrane. Trim bottoms if needed so peppers stand upright.
Optional: Blanch peppers in boiling salted water for 3–4 minutes. Drain cut-side down.
Heat olive oil in a skillet over medium heat. Sauté onion for 5–6 minutes until soft. Add garlic and cook 1 minute.
Add ground beef. Cook over medium-high heat for 6–8 minutes until fully browned with no pink. Drain excess fat, leaving about 1 tbsp.
Add tomato paste and stir for 1 minute. Add crushed tomatoes, cumin, paprika, salt, and black pepper. Cook for 4–5 minutes until sauce thickens.
Remove from heat. Fold in cooked rice and parsley. Taste and adjust seasoning.
Arrange peppers upright in a baking dish. Fill each one to about 1cm above the rim. Pour water or stock into the bottom of the dish.
Top each pepper with shredded mozzarella. Cover tightly with foil and bake for 25 minutes. Remove foil and bake a further 10–15 minutes until cheese is golden and pepper walls are tender.
Rest 5 minutes before serving.
