In a mixing bowl, mash the canned tuna into small pieces. Mix in the Japanese mayonnaise and wasabi. Combine well and set aside.
Place a plastic wrap on a table and place about 2/3 cup of rice on the wrap, spread in the middle forming a rice bed.
Add 1 to 1/2 tbsp of the tuna mayo mixture into the middle of the rice bed and add another 1/3 cup of rice on top, completely covering the tuna mayo mixture.
Lift the edges of the plastic wrap towards the middle while gently forming a rice ball and closing the plastic wrap. Gently form rice ball into a triangle. Take off the plastic wrap and set onigiri balls aside.
Cut nori sheets into 2 x 7 in strips. Wrap each onigiri with a nori strip and sprinkle with some Furikake or Japanese seasoning.