Combine the onions, garlic, soy sauce, gochujang, brown sugar, honey, rice vinegar, and sesame oil in a bowl and mix well. Place the chicken drumsticks and thighs into a large zip-top bag and pour the marinade in, making sure all of the chicken is covered. Seal the bag and lay it flat in a large, shallow pan or on a plate. Marinate the meat in the refrigerator for 6 to 8 hours or overnight, flipping once halfway through.When ready to cook, prepare a high-heat grill. Remove the chicken from the marinade and lay the chicken pieces skin-side down on the grill. Grill for a few minutes on each side to sear the outside.Once you have a good sear, lower the grill's heat to medium-low. Cook the chicken with the lid closed for 15 to 20 minutes, flipping them every 3 minutes so they cook evenly. When finished, the chicken should be cooked through, no longer pink inside, and register 165°F in the thickest part of the meat with an instant-read thermometer. Serve immediately.