Remove skin and fat from the wels catfish fillet. Finely crush mustard seeds, coriander seeds, and peppercorns and mix with the salt and sugar.

Distribute a third of this mixture in a thin layer at the bottom of a bowl. Place the wels catfish fillet on top and sprinkle with thyme. Apply the rest of the mixture to the top and sides of the fillet and rub in well. Finally, sprinkle on the rum and marinade chilled for ca. 36 hours.

Remove the fish from the marinade, cleaning off any marinade that remains, and slice as thinly as possible.

Toss Belgian endive briefly in rapeseed oil and season with herb salt.

Arrange fish and Belgian endive on plates along with the vinegar-marinated plums. Finish by adding fresh ground elder leaves and a bit of canola oil.