2) Place the potatoes on a baking sheet, soften the butter and liberally cover the potatoes. Season with salt and pepper and roast for 35 minutes at 200C/Gas 6.
3) Meanwhile, saute the spinach in an oiled frying pan and transfer to a mixing bowl once wilted. Squeeze out as much liquid as possible. Finely chop the walnuts and add to the bowl along with the ricotta and salt and pepper. Mix well until combined.
4) Clean and prepare each cod fillet then lay face down on a long piece of foil on top of a baking sheet. Cover with the mixture then roll tightly from the thickest end to the thinnest until the cod is pin-wheeled. Wrap the foil tightly around and place the sheet in the oven at 190C/Gas 5 for 15 minutes.
5) Allow to rest for one minute before unwrapping and serve piping hot with the potatoes.