How to Make It
In a bowl, whisk together the miso, sugar, sesame oil, fish sauce, vinegar, chile powder, garlic and ginger. Put the chicken and onion in a large, shallow dish, add half of the marinade and turn to coat completely. Cover and refrigerate for 8 to 12 hours. Refrigerate the remaining marinade. Discard the onion.
Light a grill or preheat a grill pan. Remove the chicken from the marinade and brush with oil. Grill the chicken over moderate heat, turning once, until browned and cooked through, about 10 minutes.
Stir the scallions into the reserved marinade and brush some on the chicken. Grill for 10 seconds per side, until glazed. Brush a little more marinade on the skin. Serve with lettuce leaves for wrapping.
Gochugaru is available at Asian markets and from TriFood.com.