Preheat the oven: Set to 375°F (190°C).
Line a baking dish with foil or lightly grease it.
Prep the peppers: Slice the bell peppers in half lengthwise and remove seeds and membranes. Brush lightly with olive oil and sprinkle with salt and pepper. Place cut side up in the baking dish.
Soften the peppers (optional but helpful): Pre-bake for 10 minutes to give them a head start.
This helps them turn tender without overcooking the filling.
Make the buffalo mixture: In a large bowl, combine shredded chicken, buffalo sauce, ranch or blue cheese dressing, garlic, celery, green onions, paprika, and butter if using. Stir until everything is evenly coated. Taste and add salt and pepper as needed.
Add cheese: Fold in half the mozzarella and the blue cheese crumbles (if using).
Reserve the rest of the mozzarella for topping.
Fill the peppers: Spoon the buffalo chicken mixture evenly into the pepper halves. Pack it in so each pepper is full.
Top with cheese: Sprinkle the remaining mozzarella on top of each pepper.
Bake: Return the dish to the oven and bake for 18–22 minutes, until the cheese is melted and bubbling and the peppers are tender.
Finish and serve: Let cool for 5 minutes. Garnish with extra buffalo sauce, a drizzle of ranch or blue cheese dressing, and more green onions or parsley.
Serve warm.