Combine the shoyu, mirin, sake and sugar in a small bowl and stir until the sugar has dissolved.
Lightly spray the oil in a large. heavy-based frying pan over medium-high heat. Add the chicken,
skin side down, and cook for 4-5 minutes, or until the skin is golden. Turn the chicken over and
cook for a further 3-4 minutes, or until golden and almost cooked through. Remove from the pan.
Discard any excess fat from the pan, then pour in the shoyu mixture, increase the heat to high
and bring to the boil. Cook for 1 minute, or until the liquid has slightly reduced and is glossy.
Return the chicken and any juices to the pan and turn to coat well. Remove from the pan and slice
on a slight angle into 1.5cm / 5/8 in wide strips. Holding the chicken thighs in their original
shape, transfer to a plate or bowl. Serve with rice and vegetables or a salad.