In a small bowl, mix together tuna with mayonnaise and red pepper sauce. Cut nori sheet in half and place it on bamboo mat, shiny side down. Dampen your fingers in water. Spread a thin layer of sushi rice over the seaweed, do not cover completely. Leave a 1-inch margin at the ends uncovered to seal the roll. Lay 2 tablespoons spicy tuna across rice.
To roll; slowly fold the end of the mat closest to you over the filling and tuck it in. Use medium pressure to create a compact tube. Remove the mat from around the roll, press in the loose ends and place it on a cutting board, seam side down. Using a wet, sharp knife, slice the roll in half then into 6 equal pieces. Garnish with radish sprouts. Serve with wasabi, soy sauce and pickled ginger.