1. Soak bamboo skewers (6" long) in water for 30 minutes.
2. Mix soy sauce, sugar, Sake, and Mirin in a small pot, and boil for 8-10 minutes until the sauce gets a little thick. Set aside.
3. Cut chicken thighs into 1" cubes, and cut onions (white part) into 1" long pieces. Skewer chicken and onions alternately.
4. In a heated frying pan, spread oil very thinly, and cook skewered meat at medium high heat for 5 minutes. Turn and cook another 5 minutes until browned and cooked through.
5. Immediately dip chicken in the sauce, and serve.
Yakitori with salt:
1. For Yakitori with salt, skewer just chicken pieces and sprinkle with salt. Cook in the same way as Yakitori with sauce (but don't dip in sauce).