In a bowl, combine soy sauce, mirin, sake, sugar, grated ginger, and minced garlic; whisk until the sugar dissolves.

Cut chicken thighs into 1-inch pieces and add them to the marinade, ensuring they are fully coated. Let the chicken marinate for at least 15 minutes at room temperature, or up to an hour in the refrigerator.

Heat sesame oil in a large skillet or wok over medium-high heat. Add the marinated chicken and cook for 5-7 minutes until browned and cooked through, turning occasionally.

While the chicken cooks, whisk cornstarch and water together in a small bowl to create a slurry.

Pour the remaining marinade into the skillet with the cooked chicken. Bring to a simmer, then stir in the cornstarch slurry.

Cook for an additional 2-3 minutes, stirring constantly, until the sauce thickens to your desired consistency. Remove from heat.

Garnish with chopped green onions and sesame seeds, then serve immediately with your preferred side dishes like rice or steamed vegetables.