Cut the tops off the bell peppers and remove the seeds and membranes. Lightly brush the outside with olive oil and place them upright in a baking dish.
In a large skillet, heat a drizzle of olive oil over medium heat. Add the chopped onion and sauté until translucent, about 3-4 minutes.
Add the minced garlic and ground chicken to the skillet. Cook until the chicken is browned and cooked through, breaking it up into small pieces as it cooks.
Stir in the drained diced tomatoes, cooked quinoa (or rice), black beans, cumin, chili powder, smoked paprika, salt, and pepper. Mix everything well and cook for an additional 2-3 minutes to heat through.
Remove the skillet from the heat and stir in half of the shredded cheese until melted and combined.
Stuff each bell pepper generously with the ground chicken mixture, pressing down slightly to pack it in. Top each pepper with the remaining cheese.
Cover the baking dish with foil and bake for 25 minutes. Then, remove the foil and bake for another 10-15 minutes, or until the peppers are tender and the cheese is bubbly and golden.
Remove from the oven and let them cool slightly before serving. Garnish with fresh cilantro.