Cut the top off of the large onion to create a small lid and hollow out the inside with a spoon. Stir together the chopped chicken breast, chopped scallions, minced garlic, teriyaki sauce, sesame seeds, sugar, and pepper and spoon the mixture into the hollowed-out onion. Place the lid on top of the onion and transfer the teriyaki chicken onion bomb to a muffin pan.
Position pieces of charcoal to the left and right sides of the grill, light the charcoal, and place the muffin pan in the center of the grill so that the onions get cooked from both sides. Cover the grill and cook the onion bombs for 20–30 minutes.