Gather all the ingredients.
Slice 4 cloves garlic and put them in a large container or bowl.
Peel ½ onion but don‘t cut off the root section.
Grate the onion, keeping the juice.
To the garlic, add the grated onion with juice, 4 Tbsp sake, 2 Tbsp mirin, and 3 Tbsp soy sauce. Mix together.
Prick the skin of 4 boneless, skin-on chicken thighs.
Season the chicken with half of the salt and pepper on the skin side, and the remaining on the other side.
Add the chicken to the marinade and refrigerate for 20–30 minutes.
In a non-stick frying pan, heat 1 Tbsp neutral oil on medium heat.
Add the chicken, skin side down, and pan-fry until golden brown, about 3 minutes.
Flip the chicken, lower the heat to medium low, cover slightly ajar, and cook for 6–7 minutes.
Check if the chicken is cooked through (internal temperature should be 165ºF or 74ºC).
Wipe off excess oil and add the reserved marinade into the pan.
Simmer on medium-low heat for 2–3 minutes, coating the chicken with the sauce.
Serve the chicken on a plate and pour the sauce over.