Make the marinade: In a bowl, whisk soy sauce, brown sugar (or honey), mirin (or rice vinegar), sesame oil, garlic, ginger, and optional heat. Reserve 1/4 cup for glazing later. Pour the rest into a zip-top bag or shallow dish.

Marinate the pork: Add the pork tenderloin to the marinade, coat well, and refrigerate for at least 30 minutes and up to 8 hours. Tip: Turn it once or twice so every side gets contact with the marinade.

Choose your cooking method: You can grill, roast, or pan-sear then finish in the oven. Aim for an internal temperature of 145°F at the thickest part, then rest 5–10 minutes.

For grilling: Preheat grill to medium-high. Oil the grates. Grill the tenderloin 4–5 minutes per side, rotating for even color, 12–16 minutes total. Brush with some reserved teriyaki during the last few minutes.

For roasting: Preheat oven to 400°F. Pat the pork dry and place on a foil-lined sheet. Roast 18–22 minutes, turning once. Brush with reserved teriyaki in the last 5 minutes to glaze.

For pan/oven: Heat neutral oil in an oven-safe skillet over medium-high. Sear the pork 1–2 minutes per side until browned. Transfer skillet to a 400°F oven and roast 10–14 minutes, brushing with teriyaki near the end.

Make the glaze: While the pork cooks, bring the remaining reserved teriyaki to a simmer in a small saucepan. Mix cornstarch with water to make a slurry. Whisk it into the sauce and simmer 1–2 minutes until glossy and slightly thickened.

Rest and slice: When the pork hits 145°F, transfer to a cutting board, tent loosely with foil, and rest 5–10 minutes. Slice into 1/2-inch rounds. Spoon glaze over the top, then garnish with sesame seeds and scallions.

Serve: Plate with steamed rice and vegetables. Drizzle extra glaze over everything for that classic teriyaki shine.