1. Mix together the mirin, soy sauce, and ginger in a large zip-top freezer bag. Add the chicken thighs, force the air from the bag and seal it closed. Massage the chicken to distribute the marinade then refrigerate the chicken for 8 to 12 hours, flipping it over a few times to distribute the marinade.
2. To cook the chicken, drain the chicken over a saucepan and set the pan over medium heat reduce the marinade it comes to a full boil, then reduce the heat and cook it longer, until it’s slightly boiled down.
3. Heat a grill pan or cast-iron skillet over high heat and add the chicken thighs, skin side down, cooking off only the amount of thighs in the pan so they are not crowded – I did three at a time in mine.
4. Sear the thighs for 2-3 minutes on each side, then cover, and cook for 6 minutes.
5. Remove the cover, wipe the condensation off the underside of the cover with a kitchen towel, flip the thighs over, cover, and cook for another 6 minutes.
6. Uncover the chicken and finish cooking them for about 3 minutes on each side, brushing and basting the chicken with the reserved marinade as you go, until the skin and meat have darkened.
Remove from the pan and cook off the remaining chicken thighs the same way.