Brush 4 sheets of filo with a little of the oil, then cover with the remaining sheets. Pile a quarter of the prawn mixture along the narrow edge of each sheet, fold over the edges, then roll up to give you 4 large spring rolls. These can now be frozen for up to 1 month; defrost thoroughly before cooking as below. Brush with the remaining oil, sprinkle with sesame seeds if using, then bake on a baking sheet, seam-side down, for 15 mins until golden and crisp. Serve with a green salad, and a good dollop of extra chilli sauce on the side.