Rinse the meat with cold water, pat dry and cut across the meat fiber into medallions about 3 cm thick.
Heat butter and oil in a pan and fry the fillets for about 4 minutes on each side. Take out of the pan and keep warm.
In the meantime, mix the cream, mustard and vinegar in a bowl. Add the chopped gherkins. Lightly sauté the diced shallots in the frying fat. Pour in the cream mixture and simmer for about 4 minutes, stirring constantly.
Arrange the fillets on a platter. Pour the sauce over and serve.