Spread the pepper evenly on a tray and roll the tuna fillet in the pepper to form a fairly even crust, set aside. Cook on a griddle pan for 4 minutes. For the Ponzu sauce: Combine all the ingredients into a medium sized pan, bring just to the boil, remove from the heat and serve. To make the potato gallettes: Barrel the potatoes evenly and slice finely. Evenly place the potato slices with hot oil in egg rings and saute until golden brown and matted. For the greens, heat the oil in a large pan. Wilt the spinach leaves, stirring constantly for 30 seconds and season. To serve: Place a potato gallette on a large plate, centre some wilted spinach on top. Carve the tuna into 4 equal portions and finish with the ponzu sauce.