Rinse the cod, pat dry and season with salt and pepper. Peel and wash the potatoes and cut them into large pieces. Peel the parsnips and cut into pieces of similar size. Wash the lemon, rub the peel with a fine grater and quarter the lemon. Beat the egg and whisk with a fork in a deep plate. Sprinkle the sesame seeds evenly on a large, deep plate.
Cook the parsnips and potatoes in salted water for about 20 minutes. Drain the potatoes and parsnips and roughly mash them with the pounder. Thoroughly stir in the milk, butter and lemon zest. Season the mash to taste with salt, pepper and nutmeg
First pull the cod through the egg and then turn it in the sesame. Heat the oil in the pan and fry the fish in it for about 3 minutes on each side.
Arrange the cod and mash on plates and sprinkle with a little cress. Place lemon wedges with the cod and serve immediately hot.