Peel, wash and coarsely chop the potatoes. Cook in salted water for about 20 minutes.
For the gratin, wash parsley, shake dry and chop finely. Peel onion and chop finely. Mix with strained tomatoes and parsley. Season with salt and pepper.
Wash the cod, dab dry and season with salt and pepper. Place in a flat greased casserole dish. Spread tomato sauce around it and a little bit over it. Sprinkle with breadcrumbs. Spread 40 g butter in pieces over it.
Bake in the preheated oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer) for approx. 20 minutes.
For the puree, drain the potatoes. Add milk and 40 g butter. Mash everything finely. Season the puree with salt, pepper and nutmeg. Arrange everything.