Mix the butter with the lemon zest, season. Set aside 1 tbsp of the mixture. Mix the rest with the cress. Transfer the mixture to a piece of clingfilm, shape into a roll, freeze for approx. 1 hr. Approx. 10 mins. before serving, cut the butter into slices approx. 7 mm thick.
Brush the sheets of foil with the reserved lemon butter (leaving a border of approx. 3 cm free all around). Place one sheet of foil on the baking tray, greased side up Salt the asparagus and spread over the foil. Place the second sheet of foil on top, greased side down. Tightly fold over the edges of the foil several times.
Bake for approx. 30 mins. in the centre of an oven preheated to 180°C.
Heat the clarified butter in a non-stick frying pan. Fry the fish in batches over a medium heat for approx. 3 mins. on each side, season with salt.
Serve the cod fillet with the cress butter and asparagus.