We have strong traditions when it comes to celebrations and food. On New Year's Eve we eat boiled cod with mustard sauce and a list of condiments that varies from region to region. The boiled cod and mustard sauce, however, are basic. The lightness of fish is appealing after several days of heavy Christmas eating.

As a child I always looked forward to the New Year's Eve's dinner, particularly when it was celebrated with my farfar (grandad on your father's side) on Ærø, a small island south of Funen, where my father's family comes from. The reason why new year was so exciting was my Aunt Sarah's excellent cooking. She is now 97 and lives in the same house.

For new year celebrations a big part of my family went to Aunt Sarah's house – not to my parents', because they were young and partying in Copenhagen. I remember my excitement when I entered the house and recognised the smell of boiled cod – a smell I still love. It brings me right back to that silent atmosphere and to seriously good eating.

The table would be very simply set with a white tablecloth, silver and the best crystal glasses; all the red Christmas colours would be gone. When we sat down to eat, there were no starters, but all the condiments would be placed in small bowls around the table. My aunt would carry in the large fish on a serving platter. There it was: the big new year cod, complete with skin and head. She would place it in the middle of the table for everyone to view, and then she would carve it, remove the skin and cut out white, shiny, flaky pieces of cod and place them on our plates.

Aunt Sarah, who came from the Faroe Islands, could come across as a bit scary to a child, but I always felt so at home and safe around her. I don't doubt it was because of all the love that she put into her cooking.

Cod with mustard sauce

Serves 6
For the cod
whole cod 1, about 3 kg

For the brine
water 3 litres
white wine 500ml
salt 1 tbsp
shallot 1
bay leaves 2
peppercorns 15

For the potatoes and condiments
bacon 200g, diced
beetroot 300g, grated
fresh horseradish (or grated horseradish in brine) 100g
organic eggs 6
potatoes 1 kg

For the mustard sauce
butter 30g
plain wheat flour 2 tbsp
Colman's mustard powder, ready-made or Dijon 1 tsp
Dijon mustard 3 tbsp
fish broth, from cooking cod 500ml
double cream 100ml

Prepare the condiments. Fry the bacon until it's crisp and golden. Grate the beetroot to medium coarseness and season it with salt and pepper. Grate the horseradish finely. Boil the eggs and chop them finely so that the mixture becomes yellow throughout. Boil the potatoes until tender, peel them and keep them warm.

Place the cod in a large pot or a fish kettle with water and all the ingredients for the brine. Bring it to the boil, then reduce the heat and leave to simmer gently for 15 minutes. Turn off the heat and leave the fish and the water in the pot for 10 minutes while you prepare the sauce. Test if the cod is done by gently pulling the back fin; if it loosens, the fish is done.

Now prepare the mustard sauce (if you are not keen on mustard sauce, you can serve the cod with brown butter). Melt the butter in a small saucepan over a low heat, then add the flour and stir until it forms a smooth paste that comes away from the sides of the pan. Strain 500ml of the cooking liquid from the cod, then add it gradually to the pan, stirring well after each addition so that no lumps form as the sauce thickens. Add the mustard powder, mustard and cream, stir again until the sauce is smooth and just coming to the boil – don't let it boil as the mustard can turn bitter. Season to taste with salt and pepper and more mustard if you like it spicier, then remove from the heat.

Lift the cod out of the pot and place on a serving dish ready to carve at the table. Put the potatoes, eggs, bacon, beetroot and mustard sauce in dishes for people to add to their plates.

Note: If you don't want to buy a whole fish you can buy a fillet; 800g will serve 4 people. Buy it with the skin on. Use 1 litre of water and 200ml white wine to boil it in, cooking for 5-7 minutes and resting for 10 minutes.

Scandinavian Christmas by Trine Hahnemann is out now (Quadrille). To order a copy for £13.59, with free UK p&p, go to theguardian.com/bookshop or call 0330 333 6846