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 tablespoon soy sauce, 2 tablespoon brown sugar, 1 tablespoon mirin, 3 cloves garlic, 2 teaspoon minced ginger, 2 teaspoon sesame oil, ¼ teaspoon salt, Freshly ground black pepper