Combine tamari, sugar, water, garlic, and ginger in a large stock pot or Dutch oven. Heat over medium heat until the sugar dissolves.

Add chicken thighs, bring to a boil, reduce heat, and simmer uncovered, turning every 5 minutes until the chicken is almost cooked through, an internal temperature of 155°F.

Combine cornstarch with 2 tablespoons of water, stir into the sauce, raise the heat back up and continue to cook until the sauce is thick and glossy, and the chicken is 165°F.

Put rice on a platter, top with chicken and sauce, garnish with green onions and sesame seeds.