Toss the chicken pieces with 1 tablespoon soy sauce and the grated ginger. Let them sit while you build the rest, at least 10 minutes.
Make the nanban sauce. Stir the rice vinegar, 3 tablespoons soy sauce, sugar and mirin together in a wide bowl until the sugar dissolves. Set it near the stove so you can dunk the chicken the moment it comes out of the oil.
Make the tartar. Combine the Greek yogurt, mayo, chopped hard boiled eggs, pickle and squeezed onion. Season with a pinch of salt and pepper. Keep it cold until serving.
Coat the chicken. Beat the egg into the marinated chicken, then dredge each piece in potato starch, pressing so it sticks in a thin even layer. Shake off the excess.
Heat the oil in a large skillet over medium-high until a pinch of starch sizzles on contact. Fry the chicken in two batches, about 4 minutes a side, until deep golden and cooked to 165F in the center. Do not crowd the pan or the crust steams instead of crisping.
Straight out of the oil, drop each piece into the nanban sauce and turn it once to coat. Lift it out so it stays crisp and does not sit and go soft.
Pile the chicken on plates, spoon the cold tartar over the top and scatter with chives. Serve with rice and shredded cabbage.