Coat the meat thickly with mustard, place in a well-sealed baking dish and marinate in the refrigerator for 2 days with the lid closed.
Place the casserole dish with the roast in the cold oven, set to 180 degrees. As soon as the mustard turns a little light brown here and there, turn down to 80 degrees on the spot. Let the roast stand for 3 to 3 1/2 hours, check the cooking state with a meat thermometer.
Then, remove the roast, put the mustard and stock in a small pan, add a dash of Noilly Prat, bring to the boil, mix with sour cream, bring to the boil, season with pepper.
Cut the meat into thin slices and arrange on a warmed plate, pour a little sauce over it. Serve the rest of the sauce separately.