In a shallow dish, combine black and white sesame seeds and stir to mix. Season tuna with salt and pepper and dredge in sesame seeds, coating evenly.

In a searing pan over high heat, warm oil until smoking. Arrange tuna in pan (do not overcrowd) and cook until white sesame seeds start to turn golden underneath, about 1 min. Turn tuna and cook until white sesame seeds start to turn golden underneath, about 1 min.

Transfer tuna to a cutting board and cut into 1/4-inch-thick slices. Arrange tuna on individual plates, overlapping slices. Serve immediately.