The secret is in preparing the fish. Bleed it first. Clean it, discarding all but the two shoulder loins. Carefully cut away sinews and liver areas, keeping only the best meat. Chop into half-inch cubes. Cook rice according to package directions. Cut nori sheets in half with scissors. Combine mayonnaise, chili sauce, soy sauce, wasabi, and sesame oil in a small bowl. Add tuna. Roll nori into cones; plug end with a spoonful of rice; add alternate layers of tuna and rice, ending with tuna. Dip in soy sauce, chili sauce, and wasabi. Serve with warm sake -- and plenty of napkins!