Heat a frying pan, add some vegetable oil, and cook the thinly sliced onions. Cook slowly over medium heat to prevent burning them. Sauté just enough for to lightly brown.
Place the chicken thighs in the pan, skin facing down, on top of the onions.
Cover and cook for 15 minutes over low heat. Moisture will be released from the meat and onions to steam-cook them.
Pour in the sake and soy sauce. Use a wooden spatula to mix, taking care not to burn the onions. Remove the lid and after 5 minutes over medium heat, turn the meat over. Simmer in the juices.
Remove the meat and cut into bite-sized pieces. Transfer to a serving plate. Top with the juicy onion sauce, and garnish with chopped leek to taste.
Another user reports that if you slice the onions too thinly, they'll burn easily.
Story Behind this Recipe
My aunt taught me this simple recipe 25 years ago. It can be made in just one frying pan and goes well with white rice.
Be sure to start cooking the chicken with the skin facing down. It'll make the sauce juicy and the meat will absorb the flavors. This doesn't use sugar, but the sweetness of the onions brings out richness in the delicious sauce. I measured the amounts of ingredients to post the recipe, but I always just eyeball them.