In a small bowl whisk together soy sauce, rice vinegar, 1 teaspoon sesame oil, grated ginger, and honey until blended, then stir in sliced green onion; set aside.

Remove tuna from the refrigerator 15–20 minutes before cooking and pat completely dry with paper towels.

Brush tuna steaks on all sides with the 2 tablespoons sesame oil, then season evenly with kosher salt and black pepper.

Combine black and white sesame seeds in a shallow dish; press each tuna steak firmly into the mixture to coat all sides.

Heat a cast-iron or heavy stainless skillet over high heat, add vegetable oil, and heat until the oil shimmers and a faint smoke appears, about 2–3 minutes.

Carefully lay tuna steaks in the hot pan and sear the first side without moving for 45–60 seconds.

Flip tuna with tongs and sear the second side for another 45–60 seconds, then quickly sear each thin edge for about 15 seconds per edge.

Transfer seared tuna to a cutting board and let rest 1 minute.

Slice tuna against the grain into 1/4-inch thick pieces and serve immediately with the prepared dipping sauce.