Bring the wine and sugar to the boil, and then add the beef stock and the blackcurrants. Cook for about ten minutes, then sieve and season with Dijon mustard and salt. Remove the sauce from the heat and, while still hot, stir in the cold cubed butter to create a creamy, velvety sauce. Fry the steaks in a little fat in a hot frying pan until they are done to your taste. If possible, do not cook more than medium, otherwise the meat will lose its flavour intensity. Season with salt and pepper, and serve with a light salad or a baked potato.