How to Make It
In a bowl, mix the soda, soy sauce, vinegar, grated onion, ginger, garlic and sugar. Add the chicken; turn to coat. Let stand at room temperature for 1 hour or refrigerate overnight.
Remove the chicken from the marinade and pat very dry. Reserve 1 cup of the marinade. Heat 1 1/2 tablespoons of the oil in a large, deep skillet. Add the chicken skin side down and cook over moderately high heat, turning once, until browned and cooked through, 7 to 8 minutes. Transfer the chicken to a plate.
Wipe out the skillet and heat the remaining 1 1/2 tablespoons of oil. Add the sliced onion and cook over moderate heat, stirring, until browned, 5 minutes. Add the reserved 1 cup of marinade; boil until slightly reduced, 2 minutes. Swirl in the butter and add the cilantro. Return the chicken and any juices to the skillet and cook until heated through, 1 minute. Serve with steamed rice and hot sauce.