Prep the oven and peppers: Preheat oven to 400°F (200°C). Brush pepper halves with olive oil and a pinch of salt and pepper.
Place cut side up in a baking dish.
Soften the peppers (optional but recommended): Bake empty pepper halves for 8–10 minutes to jump-start cooking. This keeps them tender but not soggy.
Make the buffalo chicken filling: In a large bowl, stir together buffalo sauce, Greek yogurt, and cream cheese (if using) until smooth. Add garlic powder, onion powder, smoked paprika, a pinch of salt, and black pepper.
Fold in chicken and veggies: Add shredded chicken, celery, and red onion.
Mix until evenly coated. Taste and adjust heat with more buffalo sauce if you like it spicier.
Fill the peppers: Spoon the mixture into each pepper half, mounding slightly. Top with mozzarella and a sprinkle of blue cheese if using.
Bake: Return to the oven for 15–18 minutes, until cheese is melted and peppers are tender but hold their shape.
Finish and serve: Let cool 3–5 minutes.
Garnish with chopped parsley or chives. Add a light drizzle of buffalo sauce or ranch, or a squeeze of lemon for brightness.