1. For the mayonnaise, stir together mayo, wasabi, soy sauce, lime juice and sugar. Taste and adjust seasonings if needed.
2. For the tuna, pat the fish dry with paper towels and season with salt. Put the sesame seeds on a plate and press both flat sides of the tuna steaks on the seeds to coat.
3. Heat a large, heavy skillet over medium-high heat until hot, about 2 minutes. Add the oil and swirl the pan to distribute, then add the tuna steaks, in batches if necessary to avoid crowding the pan, and sear for about 2 minutes on each side, until just the outside ¼ inch is cooked, but the center is still translucent.
4. Transfer the steaks to a cutting board and slice. Make tacos with the tuna, mayo, tortillas, scallions, radish sprouts or cilantro, and sliced avocado.
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