Preheat the pellet grill to 275°F. Cut the tops off the bell peppers and remove the seeds and membranes. Set the pepper halves aside.
In a large bowl, combine the ground beef, cooked quinoa, egg, panko, diced onion, minced garlic, 2 tablespoons of the barbecue sauce, smoked paprika, salt, and black pepper. Mix gently until just combined.
Divide the meatloaf mixture evenly among the bell peppers. Set the filled peppers upright in a grill-safe pan, leaving a little room between them.
Place the pan on the grill and smoke until the filling reaches 160°F in the center, about 1 1/2 to 2 hours. Check the temperature with an instant-read thermometer.
Brush the remaining barbecue sauce over the tops of the filling. Sprinkle the cheddar over each pepper and continue cooking for about 10 minutes, until the cheese melts.
Carefully remove the pan from the grill. Let the peppers rest for 5 minutes before serving.