Preheat the oven to 400°F (200°C). Line a baking sheet or a 9×13 pan with foil for easy cleanup. A light oil spray helps prevent sticking.
Prep the peppers. Slice bell peppers in half lengthwise, remove seeds and membranes. Place cut-side up on the pan. Season lightly with salt and pepper.
Soften the peppers (optional but recommended). Bake the empty pepper halves for 10 minutes to jumpstart tenderness. This prevents crunchy edges with a mushy filling situation—no thanks.
Mix the filling. In a large bowl, combine shredded chicken, buffalo sauce, ranch or blue cheese dressing, garlic powder, onion powder, celery, green onions, and butter (if using). Stir until the chicken is evenly coated and glossy. Taste and adjust salt/pepper.
Add the cheese. Fold in half the mozzarella and half the cheddar. This melts inside; the rest goes on top for that golden finish.
Stuff generously. Spoon the buffalo chicken mixture into each pepper half, mounding slightly. Don't be shy—full peppers = happy eaters.
Top with remaining cheese. Divide evenly over the peppers. Press lightly to help it adhere.
Bake. Return to the oven for 15–20 minutes, until the peppers are fork-tender and the cheese is melted and bubbling with a few golden spots.
Finish strong. Drizzle with extra buffalo sauce, ranch or blue cheese dressing, and sprinkle with blue cheese crumbles or more green onions if you like. Rest 5 minutes so the filling sets (and you don't scorch your tongue).
Serve. Pair with a crisp salad, carrot sticks, or roasted potatoes. Then accept compliments graciously.