Preheat oven to 200°C / 400°F. Line a baking sheet with parchment.
Prep peppers: Wash, halve lengthwise, and remove seeds. Arrange cut-side up on the baking sheet. Trim bottoms slightly if needed so they sit flat.
Make filling: In a large bowl, combine cream cheese, shredded mozzarella, grated Parmesan, chopped turkey pepperoni, egg, Italian seasoning, garlic powder, onion powder, salt, and pepper. Fold until evenly mixed.
Fill peppers: Use a teaspoon or small scoop to portion ~1 tbsp filling into each pepper half. Do not overfill.
Bake: Bake 12–15 minutes until filling is set and tops are lightly golden. Optionally broil 30–45 seconds for extra color.
Rest & garnish: Let rest 3–4 minutes. Garnish with chopped basil or parsley. Serve warm with sugar-free marinara for dipping if desired.
Store: Refrigerate up to 3 days or freeze for up to 3 months (see storage tips).