Place the head and tails of the fish in a large pot. Add the roughly chopped vegetables. Cover with water, bring to a boil and simmer gently for half an hour.

Strain the broth. Reserve the carrots, parsnip and bell pepper. Chop them into smaller cubes and add them to the finished soup at the end of the cooking time.

Strain the broth again through a finer sieve lined with muslin cloth. Clean the pot and pour the fish broth back into the pot.

Peel the potatoes but leave them whole. Only halve them if they are large (and use only 6 potatoes if they are larger). Add them to the pot and bring to a boil.

Start chopping the onion, carrots and bell pepper used for the soup into small cubes and add them to the soup as soon as they are chopped.

Leave to simmer gently for about 15 minutes or until the potatoes are almost done.

Halve the larger fish, like the trout and mackerel. Add all the fish pieces to the soup together with 1-2 teaspoons salt (to taste) and the vinegar. Start with 2 tablespoons vinegar and add more to taste. It is important to add the salt and vinegar together with the fish to the pot, it will prevent the fish from falling apart.

Leave to simmer for 5 to 10 minutes or until cooked but still tender and not overcooked. The fish will continue cooking in the soup after you turn off the heat, so don't worry about the fish being still uncooked.

While the fish is still simmering, whisk together the egg and the smetana or crème fraiche. Slowly add two ladles hot soup to the egg mixture while whisking all the time. This will temper the mixture, so that it will not curdle when you add it to the soup. Pour the egg smetana mixture slowly into the soup while whisking. Take off the heat immediately.

Add the reserved vegetables from the broth to the soup. Adjust the taste with salt, pepper and more vinegar, if necessary. Chop the lovage and add it to the soup just before serving.

Grate the garlic cloves into a small bowl.

Add a pinch of salt and slowly whisk in the vegetable oil.

Add about 125 ml/ ½ cup cold water and mix well.

If the sauce is too strong for your taste, you could add a bit more water.

Lift the fish and the potatoes from the soup and place them on a serving platter.

Serve the soup with some garlic sauce and more vinegar, if you like, as a first course and the fish and the potatoes with more garlic sauce as a second course.

Or serve everything together as a single course.