1) In a non-reactive bowl, combine soy, honey and wasabi powder. Reserve 1/4 cup for dipping sauce. Roll each piece of tuna in this mixture to coat evenly. Marinate from 1 hour to overnight. Remove the tuna from the marinade and discard the marinade.
2) On a plate, lay the sesame seeds. Roll the tuna in the seeds to evenly coat.
3) Fire up the chimney and top with a well-oiled grate. Sear for 15 to 30 seconds per side or to desired temperature. Remove to rack and rest for 3 minutes. Cover with foil or cling film to achieve carry over cooking.
Slice tuna thinly and serve with the dipping sauce.