With a sharp knife, chop the tuna into ½ inch dices and place in a bowl.
Add mango, green onions, sesame oil, grated ginger, ponzu sauce, chili powder and a splash of soy sauce and mix well.
In a small round mold place the tuna mix in the button half and add the jasmine rice on top half. Press firmly to make sure ingredients won't fall apart.
Flip over a serving dish and gently retreat the mold out. Sprinkle sesame seeds and decorate with some green onion cut in julienne.