Pour the fish stock or water into a large soup pot and bring to a rolling boil over medium-high heat. Add the whole peeled onion, sliced carrots, bay leaves, and black peppercorns to the pot.

Reduce the heat to a gentle simmer and cover the pot. Let the vegetables and spices cook for about 10 minutes to infuse the broth with deep, rustic flavors.

Add the diced potatoes to the simmering broth, stirring gently to combine. Continue to cook for another 10 to 12 minutes, or until the potatoes are fork-tender but not falling apart.

Carefully slide the fish chunks into the pot, ensuring they are submerged in the liquid. Simmer gently for 5 to 7 minutes, until the fish is entirely opaque and flakes easily with a fork, being careful not to stir vigorously so the fish stays intact.

Remove the pot from the heat, then carefully fish out and discard the whole onion and bay leaves. Stir in the vodka if using, and season the broth generously with salt and freshly ground black pepper to taste.

Ladle the hot soup into warmed bowls, ensuring an even distribution of the tender fish and root vegetables. Garnish generously with fresh chopped dill and serve immediately alongside crusty rye bread.