Put the fish roe with the parsley root, the carrot, the onion, the peppercorns, the bay leaf and the vinegar in water, and cook over low heat, barely bubbling, for 20 minutes.
Lift out the fish roe, and strain the cooking liquid.
In another pot, cook the sauerkraut with its juice and enough water to cover it, until tender, about 15-20 minutes.
Make a roux from the lard and the flour, take it off the heat and stir in the paprika, then immediately pour in a ladleful of water and stir it into the cabbage soup.
Pour in the strained fish roe cooking liquid too, and bring to a boil.
Add the fish roe to the soup, warm through, and serve hot.
Based on: Malatinszky Fanny szakácskönyve (a classic Hungarian cookbook), modernised. Calories and protein are estimated.