Preheat oven to 400°F. Halve the peppers lengthwise, remove seeds and membranes, and place cut-side up in a large baking dish. Brush the insides lightly with olive oil. Pre-bake for 12 minutes.
While the peppers roast, cook the ground beef in a large skillet over medium-high heat until browned, 5–6 minutes. Drain excess grease.
Add the diced onion and cook 3 minutes until softened. Add the garlic and cook 30 seconds more.
Stir in the tomato sauce, cooked rice, Italian seasoning, garlic powder, paprika, salt, and pepper. Simmer for 2–3 minutes until combined and thick. Remove from heat and stir in ¾ cup mozzarella.
Remove the pre-roasted peppers from the oven. Spoon the filling generously into each pepper half. Top with the remaining ¾ cup mozzarella.
Return to the oven and bake uncovered for 18–20 minutes, until cheese is melted, browned in spots, and the peppers are fully tender.
Rest 5 minutes, then scatter parsley over the top and serve.