Preheat the oven: Heat the oven to 400°F (200°C).
Prepare the peppers: Lightly grease a large baking dish or oven-safe skillet. Arrange the halved, seeded bell peppers cut-side up.
Make the filling: In a large bowl, combine the cooked shredded chicken, mayonnaise, buffalo hot sauce, garlic powder, onion powder, salt, black pepper, nutritional yeast, and sliced green onions. Mix thoroughly. Taste and adjust the hot sauce or salt if needed.
Fill the peppers: Spoon the buffalo chicken mixture into each pepper half, packing the filling gently so the peppers are generously stuffed.
Bake: Cover the baking dish with foil and bake for 30 minutes. Remove the foil and continue baking for about 20 minutes, or until the peppers are tender and the filling is bubbling and lightly browned.
Serve: Drizzle with dairy-free ranch dressing and garnish with sliced green onions and fresh herbs. Serve warm.