Preheat your oven to 400°F (200°C).
Line a baking sheet or 9x13-inch baking dish with parchment or lightly grease it.
Prepare the peppers: Slice bell peppers in half lengthwise and remove seeds and membranes. Place them cut-side up on the tray and lightly spray with oil. Season with a pinch of salt and pepper.
Par-bake the peppers: Bake for 10–12 minutes to soften slightly.
This helps them cook through without getting watery later.
Make the filling: In a large bowl, mix cream cheese, buffalo sauce, ranch (or blue cheese dressing), garlic powder, onion powder, and butter if using. Stir until smooth.
Add chicken and mix-ins: Fold in shredded chicken, half of the mozzarella, green onions, and celery. Taste and season with salt and pepper.
Add more buffalo sauce if you like extra heat.
Stuff the peppers: Spoon the buffalo chicken mixture evenly into each par-baked pepper half. Top with remaining mozzarella and crumbled blue cheese (if using).
Bake: Return to the oven for 15–18 minutes, until the cheese is melted and bubbly and the peppers are tender but still hold their shape.
Finish and serve: Let cool for 5 minutes. Garnish with extra green onions.
Drizzle with a little ranch or blue cheese dressing if you like.