Prepare the sauce: In a small bowl, whisk together soy sauce, mirin, sake, sugar, ginger, and garlic.
Cook the chicken: Heat oil in a pan over medium-high heat. Season chicken lightly with salt and pepper, then sear until golden and cooked through (6–7 minutes).
Add sauce: Pour the teriyaki sauce mixture into the pan. Let it simmer 3–4 minutes until slightly reduced.
Optional thickening: Stir in cornstarch slurry and cook 1–2 minutes until sauce is glossy and coats the chicken.
Serve: Sprinkle with sesame seeds and spring onions. Serve hot over steamed rice.