Cut the fillets into approx. 3 cm thick slices. Heat butter and oil in a pan and fry the meat for a maximum of four minutes on each side. Salt and pepper, remove from the pan and keep warm.
Peel the onion and chop finely. Finely dice the pickles as well. Sweat the onion in the frying fat, then add the cucumber cubes, cream, mustard and vinegar. Let everything simmer for about four minutes while stirring, season to taste. Put the meat back in the sauce and serve.