Preheat the oven: Set to 400°F (200°C). Line a baking dish with foil or lightly grease it.
Prep the peppers: Slice bell peppers in half lengthwise and remove seeds and membranes.
Place cut-side up in the baking dish. Sprinkle lightly with salt and pepper.
Soften the peppers (optional but recommended): Bake the empty pepper halves for 10 minutes. This helps them soften and release water before stuffing.
Sauté the veggies: Heat oil in a large skillet over medium heat.
Add onions and mushrooms with a pinch of salt. Cook 5 to 6 minutes until softened and most liquid evaporates. Stir in garlic for 30 seconds.
Cook the protein: Add ground turkey or chicken to the skillet.
Break it up and cook until no longer pink, about 6 to 8 minutes. Season with Italian seasoning, salt, pepper, and crushed red pepper.
Add sauce and mix-ins: Stir in the pizza sauce, chopped turkey pepperoni (if using), and olives. Simmer for 2 to 3 minutes to thicken slightly.
Taste and adjust seasoning.
Fill the peppers: Remove par-baked peppers from the oven. Spoon the meat sauce into each pepper half, packing it in. Top with mozzarella and a sprinkle of Parmesan.
Bake: Return to the oven for 12 to 15 minutes, until cheese is melted and bubbly and peppers are tender.
Finish and serve: Let rest for 5 minutes.
Garnish with fresh basil or parsley. Serve hot with a side salad or roasted veggies.