Combine ginger, mirin, and soy sauce and marinate the fish for 30 minutes. Preheat oil in a deep-fryer to 350 degrees F.
Mix together batter ingredients.
When preparing an order, drain and dredge fish cubes in seasoned flour. Coat fish with the batter and deep-fry until golden brown.
In a bowl, gently toss one portion of fried fish with 4 ounces of hot sweet and sour sauce. Serve immediately on a warmed dinner plate.