Make the marinade. In a large bowl, whisk together soy sauce, gochujang, brown sugar (or honey), mirin, rice vinegar (if using), sesame oil, garlic, ginger, grated onion, and scallions. Stir in gochugaru (if using) and grated pear (if using).
Prep the chicken. If using thighs, pound to an even thickness with the heel of your hand or the bottom of a small saucepan. This ensures even cooking and better contact with the marinade.
Marinate. Add chicken to the bowl, toss until well coated. Cover and refrigerate at least 30 minutes (4 hours is better; overnight is great. If using pear, add it during the last hour only and don't exceed 12 hours total).
Cook/Skillet method. Heat a large skillet or cast iron pan over medium-high heat and add the cooking oil. Lift chicken from the marinade, letting excess drip off. Cook in batches (don't overcrowd) for 4 to 5 minutes per side, until caramelized and cooked through (165°F internal).
Optional: reduce the sauce. Pour leftover marinade into a small saucepan and simmer 3 to 5 minutes until slightly thickened, stirring often. Spoon over cooked chicken.
Serve. Serve over rice or in lettuce or perilla leaves. Top with sesame seeds, scallions, and cucumbers. Add kimchi on the side if you've got it.
