Whisk together the soy sauce, mirin, rice vinegar, sesame oil and black pepper. Lightly brush the tuna with the marinade and let it rest 5–10 minutes only — don’t over-marinate.
Get the grill ripping hot — high heat is essential. Oil the grates well with neutral oil.
Lay the tuna on the grill and sear 45–60 seconds per side for rare, or up to 90 seconds per side for medium-rare. You want bold grill marks with a cold, ruby center.
Rest 2 minutes, then brush lightly with any remaining marinade.
Shower with furikake and fresh chives. Add a squeeze of lemon or yuzu if desired.
