Prepare Peppers: Preheat your oven to 375°F (190°C). Cut the tops off the bell peppers and remove the seeds and membranes. Place the peppers upright in a baking dish and bake the peppers unstuffed for 10 minutes.

OPTIONAL STEP: If you want to cook the peppers quicker, do this step instead of baking the unstuffed peppers in the oven first. Fill a large pot with water and bring it to a boil over high heat. Place the peppers in the boiling water and let them cook for 3-4 minutes, or until slightly softening. Use tongs to remove the peppers from the boiling water and place them on a plate lined with paper towels to drain any excess water. Add to a baking dish upright.

Make Chicken Mixture: In a skillet, heat olive oil over medium heat. Add diced onion and cook until softened about 3-4 minutes. Add minced garlic and cook for another minute until fragrant. Add the shredded chicken to the skillet along with the green chilies and buffalo sauce. Stir to combine and let it simmer for a couple of minutes until heated through—season with salt, pepper, and onion powder. Remove the skillet from heat and stir in half of the shredded mozzarella cheese.

Stuff Peppers: Spoon the buffalo chicken mixture into each bell pepper until they are filled to the top. Cover the baking dish with aluminum foil and bake in the preheated oven for about 20-25 minutes or until the peppers are tender and the cheese is melted.

Garnish: Once cooked, remove the foil and optionally broil the peppers for 1-2 minutes until the cheese is bubbly and golden brown. Garnish with chopped green onions and ranch dressing if desired, and serve hot.