Mix marinade ingredients together in a container large enough to hold tuna. Put tuna steaks in and turn to coat. Cover and refrigerate for one hour, turning occasionally.
Heat non-stick skillet to high heat. Sear tuna steaks 1 minute on each side.
Remove steaks from pan and slice into 1/4 inch slices on the diagonal. Sprinkle with scallion slices and sesame seeds.