Mix mascarpone with dill, salt and pepper. Lay a sheet of nori on a board and place a slice of salmon on top. Spread with a quarter of the mascarpone mixture, leaving a 3 cm (1 ¼ in) border at the end furthest from you. Roll up tightly, sealing the edge with a little cold water; use a pastry brush or your finger. Leave to rest for 20 minutes in the fridge to firm up, sitting on the seam, before cutting up with a very sharp knife.