In a large bowl, combine the chicken pieces, sliced onion, soy sauce, brown sugar, mirin, garlic, ginger, sesame oil, salt, and black pepper. Mix well to coat evenly, then marinate for 10 mins.
½ yellow onion, 3 Tbsp soy sauce, 2 Tbsp brown sugar, 1 Tbsp mirin, 3 cloves garlic, 2 tsp minced ginger, 2 tsp sesame oil, ¼ tsp salt, Freshly ground black pepper