In shallow plate, place 1 tsp tamari. On separate small plate, place sesame seeds. Place tuna steak in tamari and turn to coat on all sides. Transfer tuna to sesame seed plate, coating on all sides. Heat sesame oil on high heat. When pan is hot, add tuna, searing for about 10 seconds on 4 of the 6 sides, until sesame seeds are toasted and golden brown. (Do not sear the smallest surfaces.) Remove tuna to plate, allow to cool, and then cover and refrigerate until ready to serve.