Place onion in a small strainer and place strainer in a bowl of water for a few minutes. Squeeze out moisture. Combine chicken and onions with ginger, sugar, mirin, 1 tablespoon soy sauce and egg. Mix well. Roll into 1 1/2 inch balls, then flatten slightly.
Heat oil in a non-stick pan; brown meatballs on all sides, about 5-7 minutes.
Combine water, sake and remaining soy sauce; add to meat and cook until almost gone. Take care not to overcook.