Soak bamboo skewers in water for 30 minutes to 1 hour. (This will keep them from burning later). Mix all marinade ingredients together in a bowl. Submerge tuna cubes in marinade, cover, and refrigerate for at least 30 minutes.
Preheat a grill or grill pan to high.
Remove tuna from marinade and skewers from water, and thread tuna onto skewers. Place skewers on the edges of the grill with the longest part of the stick hanging off the edge away from the fire. Stay close by and turn the sticks by hand to keep from burning. Brochettes are done in just 3 to 4 minutes.
Sprinkle with sesame seeds before serving.
Serve over Szechuan vegetable stir-fry, or for a party, stick skewers straight out of the side of a cantaloupe half filled with melon balls.