Prepare the vegetables by cutting the carrots and celeriac into thin matchsticks (julienne) and slicing the leek into rings.

In a large soup pot, melt the butter over medium heat. Add the sliced leeks and sauté for 3-4 minutes until softened but not browned.

Sprinkle the flour over the leeks and stir constantly for 2 minutes to create a roux.

Gradually whisk in the fish stock, ensuring there are no lumps. Bring the mixture to a gentle simmer.

Add the julienned carrots and celeriac to the broth. Simmer for about 10 minutes until the vegetables are tender.

Add the chunks of cod and salmon to the pot. Reduce heat to low and let the fish poach gently for 5 minutes. Do not boil vigorously or the fish will break apart.

In a separate bowl, whisk together the heavy cream, sour cream, and egg yolks until smooth.

Temper the cream mixture by slowly adding a ladle of the hot soup broth into the bowl while whisking prevents the eggs from scrambling.

Pour the warmed cream mixture back into the main pot. Stir gently to combine and heat through for 2 minutes, but do not let it boil.

Stir in the white wine vinegar to balance the richness. Season with salt and pepper to taste.

Ladle into bowls and garnish generously with fresh chopped dill before serving.