Gather all the ingredients.
Peel and grate 1 knob ginger and crush (or mince) 1 clove garlic. In a large bowl, whisk all the ingredients for the marinade: the grated ginger, the crushed garlic, 2 Tbsp sake, and 2 Tbsp gluten-free soy sauce.
Cut the 1.1 lb boneless, skin-on chicken thighs into 5 to 6 pieces per thigh, about 1½ inches (3.8 cm) each.
Lightly season the chicken with Diamond Crystal kosher salt and freshly ground black pepper.
Add the chicken to the marinade. Coat and rub the chicken with the marinade using your hands or tongs. Set aside for 30 to 60 minutes.
In a medium bowl, combine 4 Tbsp potato starch or cornstarch and 1 Tbsp rice flour and whisk well.
In a pot, heat 2½ cups neutral oil to 340ºF (170ºC). I use a smaller 1½ QT pot, so that the oil is deep enough and I don‘t have to use much oil. Use a thermometer or wooden chopsticks to see if the oil is ready (for more details, see my post on How to Deep-Fry Foods).
When the oil is hot, drain the chicken well from the marinade and coat each piece with the potato starch mixture.
Deep-fry for 90 seconds, or until the chicken is light golden color. Work with a few pieces at a time, so the oil temperature won’t drop quickly. I worked with 3 to 4 pieces at a time.
Transfer the chicken to a wire rack to drain the excess oil. Let the chicken cook with the remaining heat while you work on the rest of the chicken.
Once the chicken is all deep-fried for the first round, use a fine-mesh sieve to pick up crumbs to keep the oil clean. If you don‘t do this step, the crumbs will get darker, which will make the oil darker.
Increase the heat and bring the oil to 350ºF (180ºC). Deep-fry the chicken for the second time for 45 seconds or until crispy and golden brown. Transfer the chicken to a wire rack to drain excess oil.
Serve the chicken immediately with lemon wedges, parsley, lettuce, shichimi togarashi (Japanese seven spice), and Japanese Kewpie mayonnaise. If you‘d like, squeeze with lemon before you eat and sprinkle shichimi togarashi for spice.