In a small bowl, combine all ingredients for dipping sauce and set aside. In a shallow dish, add sesame seeds and stir to mix if using black and white seeds.

Season tuna lightly with salt and pepper and coat top and bottom of fish completely in sesame seeds, pressing to adhere.

Place a heavy-bottomed skillet or grill pan over high heat and allow it to become scorching hot.

Add just enough oil to skillet to coat the area that the fish will occupy. Place as many pieces of tuna in the pan as will comfortably fit, but do not overcrowd the pan. Work in batches if necessary.

Cook tuna on first side for about 1 minute then flip and cook slightly longer than 1 minute on the second side. The white sesame seeds should become lightly golden brown and the fish will be cooked to rare stage.

Transfer the tuna to a cutting board and carefully slice into ¼"-thick slices. Arrange the tuna on a platter or on individual plates. Serve immediately with dipping sauce on the side.