1) Preheat your oven to 425°F (220°C). Bring a pot of water to a boil. Wash the bell peppers, slice off the tops, and scoop out the insides. If needed, trim a thin slice off the bottom so they stand upright.
2) Boil the peppers for 5 minutes to slightly soften them. (Alternatively, roast them in the oven for 20 minutes.) Transfer the softened peppers to a lightly oiled baking dish and set aside.
3) Heat the olive oil in a large pan over medium heat. Add the onions, chopped pepper tops, and mushrooms. Cook for 5 minutes, stirring occasionally, until the vegetables begin to soften.
4) Add the garlic and cook for 1 minute, until fragrant. Add the ground beef and cook for 5–6 minutes, breaking it apart as it browns, until no pink remains.
5) Stir in the ArBol sauce, paprika, oregano, basil, cooked grain, and half of the shredded cheese. Mix well until fully combined, then remove from heat. Season with salt & pepper to taste.
6) Spoon the filling into each pepper until packed. Top each pepper with the remaining shredded cheese.
7) Bake the stuffed peppers for 15–20 minutes, or until the peppers reach your preferred tenderness and the cheese is melted & bubbly. Let the peppers cool slightly, garnish with fresh parsley, and serve warm.