01 - Combine soy sauce, mirin, brown sugar, honey, rice vinegar, sesame oil, garlic, and ginger in a small saucepan. Simmer over medium heat for 5–7 minutes.
02 - Mix cornstarch with water in a small bowl. Stir into the saucepan and simmer for another 1–2 minutes until slightly thickened. Let the sauce cool.
03 - Reserve 1/4 cup of the teriyaki sauce for glazing. Place sliced beef in a shallow dish or resealable bag, pour the remaining marinade over it, cover, and refrigerate for at least 1 hour (up to 8 hours).
04 - Soak wooden skewers in water for 30 minutes if using. Thread marinated beef and scallion pieces alternately onto skewers.
05 - Preheat grill to medium-high heat. Grill skewers for 3–5 minutes per side, brushing with reserved glaze during the final minutes. Beef should be cooked through and lightly charred.
06 - Let skewers rest briefly before serving. Garnish with sesame seeds and sliced scallions as desired.