Cut chicken thighs into square pieces, about 2” (5 cm) each. Sprinkle lightly with salt and coat chicken pieces with Katakuriko (starch). Remove excess Katakuriko. Set aside.
Cut long white onions into 2” (5 cm) long pieces.
In a medium bowl, mix all the ingredients for the Teriyaki sauce until the sugar is completely dissolved.
Heat oil in a frying-pan at medium heat. Cook chicken until nicely browned on one side. Then turn over, cover with a lid, and cook until chicken is cooked through, about 3 minutes.
Add the sauce and onions, and cook for a couple of minutes. Once the sauce has thickened, remove from heat.
Put rice in a bowl, and top with chicken and the sauce. Squeeze Japanese mayonnaise over and garnish with shredded Nori roasted seaweed.