In a medium mixing bowl, whisk together the soy sauce, sesame oil, chives, shallots, furikake, lemon juice and lemon zest (if using).
Arrange the tuna steaks in a large shallow dish or resealable bags. Pour the marinade evenly over the fish, coating all sides. Cover and refrigerate for 1 hour (no longer than 90 minutes).
Preheat a grill (or cast-iron grill pan) to high heat and lightly oil the grates with neutral oil. Remove the tuna from the marinade, pat lightly with paper towels and season both sides with a little salt and pepper.
Grill about 1–2 minutes per side for rare to medium-rare, depending on thickness. Work in batches if needed to avoid overcrowding.
Slice the steaks across the grain into thick strips, or plate whole for individual servings. Sprinkle with extra furikake and chopped chives, and pair with steamed rice, a crisp cucumber salad or lightly dressed greens.
