Roll the sweet potato medallions in the flour until coated.
whisk to combine the egg yolks and the water. Sift in the flour and mix the batter until it resembles a thin pancake batter.
Fry the medallions for 6-8 minutes, rotating frequently.
Place a sheet of nori in the center of a large piece of plastic wrap. Place 1/3 of a cup of rice in the center of the sheet.
Cover the rice with cucumber slices. Top with sweet potato tempura, avocado, mayo, and sesame seeds.