Gather the ingredients.
Trim scallions; thinly slice dark green parts on an angle and reserve for garnish. Thinly slice remaining white and light green parts; place in a 10- to 11-inch high-sided skillet.
Add stock, shoyu, brown sugar, garlic, ginger, and 1 cup of the water to the skillet, whisking gently until sugar dissolves.
Add chicken thighs to the skillet, skin-side down. Bring to a boil over medium-high. Reduce heat to low; cover and simmer over low, undisturbed, until chicken is cooked through and tender, about 30 minutes, flipping chicken skin-side up halfway through cooking. Remove from heat.
Transfer chicken to a serving platter. Loosely tent with aluminum foil to keep warm. Skim and discard as much fat as possible from stock mixture in skillet.
In a small bowl, whisk together cornstarch and remaining 3 tablespoons water until well combined.
Whisk cornstarch mixture into stock mixture. Bring to a boil over medium, whisking often, and cook until sauce is thick enough to coat the back of a spoon, 1 to 2 minutes.
Remove from heat. Spoon thickened sauce over chicken and garnish with reserved scallion. Serve immediately with rice and any extra sauce.
