Place beef brisket in a roasting pan large enough for the meat and the vegetables; add juices and spices from brisket package. Combine all seasoning ingredients in small bowl; pour over brisket. Cover; bake for 2 hours.
Add the vegetables. Cover; return to oven and continue baking for 1 to 1 1/2 hours or until brisket is fork tender and vegetables are desired softness. Remove brisket and vegetables to serving platter; cover with foil and keep warm.
Beat whipping cream in chilled small bowl at high speed until soft peaks form. Gently fold in horseradish, mustard, and vinegar. Serve mustard sauce with brisket and vegetables.
Note: Prepared as directed above, the mustard sauce can be kept, covered. and refrigerated for up to three days. This mustard sauce is also good served with roast beef and pork.