Boil the potatoes in a pan of boiling salted water until soft (20-30 minutes). Mash then mix them with the scallion and enough wasabi to give the mash a bit of kick. Season with salt to taste. Let chill for 30 minute, or until the mash is very firm
Heat the oil in a skillet and saute the scallops on both sides for 2-3 minutes. Slice them thinly into 3 coin-shape pieces and season with salt to taste
Divide the mashed potatoes into 6 equal portions. Put a sheet of nori shiny-side down on a rolling mat with the longest end toward you and mound one portion of the mash on the bottom third of the nori. Spread some mayonnaise on top. Add a sixth of the scallop slices
To roll the sushi, fold the mat over, starting at the end where the ingredients are, and tucking in the end of the nori to start the roll. Keep rolling, lifting up the mat as you go and keeping the pressure even but gentle until you have finished the roll. Moisten the top edge of the nori with water to seal the sushi roll closed
Remove the roll from the mat and cut it into 4 even-size pieces on end and arrange them on a plate. Dot with some wasabi paste and sprinkle on few black sesame seeds as garnish. Repeat with the remaining ingredients