Sear the tuna:
Season the tuna fillet with the shichimi or cayenne and marinate it in the soy sauce for 5 minutes. Heat a skillet over medium-high heat. When the skillet is heated, spritz with cooking spray and sear the tuna for 1-1/2 minutes on each side. Slice the fillet against the grain into horizontal strips.
Make the rolls:
Stack the sheets of toasted nori and cut them in half, parallel to the lines imprinted on the seaweed.
Lay one sheet of nori horizontally on a bamboo rolling mat, aligned with the edge of the mat that's closest to you. Moisten your hands with a little water to keep your hands from sticking (keep a bowl of water handy). Grab a large handful of rice and toss and squeeze it lightly to form a loose oval ball.
Spread the ball of rice across the top of the nori, leaving a 1/4 band of nori uncovered at the top edge. Using both hands, "pull" the rice down to cover the nori, leaving a narrow band of nori uncovered at the bottom as well. Using your fingers, spread a pinch of wasabi in a stripe across the middle of the rice (a little goes a long way). Arrange the strips of seared tuna, the radish sprouts and the scallions in a band over the wasabi.
Moisten the exposed strip of nori farthest from you. With all eight fingers holding in the ingredients, lift the edge of the rolling mat closest to you with your thumbs. Tucking the ingredients into the middle of the roll, bring the edge of the mat over the ingredients and straight down. Leave the moistened strip of nori exposed at the top edge.
Press the roll together with your thumbs and middle fingers, while pressing down on the roll with your index fingers.
Lifting just the edge of the mat, pull it forward so that the nori roll rolls another quarter turn. The seam will now be on the bottom. Press again with fingers and thumbs, molding the roll into a squared log.
Lift the mat away and transfer the roll to a cutting surface. Dip a sharp knife into a bowl of water and let a bead of water roll down the knife's blade. Cut the roll in half and then bring one half around and cut both pieces into thirds. Stand the pieces up on a cut side.
Repeat the rolling steps four of the remaining half-sheets of nori (you'll have one left over). Serve the sushi with pickled ginger, a small dish of soy sauce, and a little mound of the remaining wasabi.