Sprinkle the prunes with the brandy or cognac 1 day before you start and leave them in the fridge to marinate. Insert a steel sharpening knife or a thin knife lengthways through the tenderloin and stuff the resulting space with the marinated prunes. Heat the butter in a frying pan, brown the tenderloin all over and season with salt and pepper. Place the stuffed veal tenderloin for a further 20 minutes in an oven preheated to 180°C (356°F) (the veal should still be slightly pink). Remove the veal tenderloin from the oven and keep it warm.

Soften the onion, thyme, bay leaf and crushed peppercorns in the butter left in the frying pan, without browning them. Add the veal gravy and any leftover marinade from the prunes and simmer for 5 minutes. Strain the gravy and season with salt and pepper. Whisk the cubes of cold butter into the gravy, one at a time.

Using a sharp knife, cut the veal tenderloin into slices. Serve with potato gratin and butter-steamed seasonal vegetables.