Put all the ingredients for the marinade in a pot, boil it for about 5 min. Leave the marinade beside to cool down a bit.
In a pan heat the oil and put the fish skinside down in the pan. Fry the fish from both sides and take it out and place it in the marinade.
If it is hot outside so leave the fish (with the marinade) in the fridge for about 2 hours.
Peel the beetroot and the potatoes and cut them into equal cubes (ca. 1 cm). Then cook them separately until they are nearly soft. (splat cooling)
Fry 2 tbsp butter in a pan, add the onions and the potatoes, let them fry for a short time and then add the beef stock. Leave it to simmer for about 10 min. until it has a creamy texture, season it with salt, pepper and a bit nutmeg, add the corned beef and leave it on the fire for about further 5 min.
In another pot heat 2 tbsp butter, add the beetroot, season with salt and pepper and heat it.
Place the potatoes ragout in the middle of the plate, add the beetroot and put the fish skin up on the top, sprinkle the cornichons around and garnish with parsley.