Cut the leeks diagonally into thick slices.
The chicken thighs should be butterflied, then held open with a fork to be salted. Also sprinkle on some potato starch and let it sit for a bit.
In a frying pan with oil, cook the chicken, first with skin side down. As the chicken begins to become a golden brown, make sure to flip it and cook both sides evenly. Afterwards, add some sake and cover with a lid.
Once the chicken is cooked all the way through, take the chicken out and saute the leeks with the leftover oil.
Add the chicken broth and 150ml of water, then add enough potato starch to thicken the water mixture.
When the onion sauce is done, replace the chicken on top of the sauce and you’re done.