Chicken Teriyaki

1.Combine the mirin and sake in a small saucepan. Heat over medium-low for 5 minutes, then add the soy sauce and sugar. Stir until dissolved. Adjust the heat to simmer for 20 to 25 minutes, until reduced by about a third and thickened. There should be a very generous 1/2 cup. Set aside to cool and concentrate. Use or store in a jar in the fridge for a week. Return to room temperature before using.
2.Rinse and pat the thighs dry with paper towel. Poke it with a toothpick or fork to let flavors penetrate. Sprinkle with salt and pepper. Heat a medium skillet over medium heat.
3.Add oil to pan and heat up briefly. Cook the thighs, smooth side down, for 3 minutes, or until a little golden underneath. Turn each piece over, lower the heat to medium-low, then cover; you want a gentle hiss. Cook for 8 minutes, or until just cooked through.
4.Add about 2 tablespoons of the sauce. Turn to coat the chicken, increase the heat to medium. Let the chicken cook in the bubbling sauce for 3 to 5 minutes, turning often, until the sauce has coated the chicken with a reddish-brown gloss. The pan will be have some fat from the chicken as well as the cooked down dark-colored teriyaki sauce.
5.Transfer the chicken to a plate and let cool for 5 to 10 minutes. Cut across the grain, arrange on a plate and garnish with a sprinkling of green onion. Serve with rice and leftover sauce.