Place the beef roast in a deep ceramic or glass bowl. Add onions, peppercorns, cloves, and bay leaf. Pour white vinegar and cider vinegar over the meat; chill, covered, for 4 days. Turn meat twice each day. Remove the meat from the marinade, dry it well with paper towels, and strain the marinade into a bowl. Reserve onions and 1 cup marinade. In a Dutch oven brown the meat on all sides in hot vegetable oil. Sprinkle meat with salt. Pour boiling water around the meat. sprinkle in crushed gingersnaps, and simmer covered for 1 1/2 hours. Turn often. Add 1 cup of reserved marinade and cook meat 2 hours or more, until tender. Remove the meat and keep it warm. Strain the cooking juices into a large saucepan. In a small bowl mix sour cream with flour. Stir it into the cooking juices and cook, stirring, until sauce is thickened and smooth. Slice meat in 1/4 inch slices; add to hot gravy. Arrange meat on a heated plater and pour extra sauce over it. Submitted By RICH BOSSARD On WED, 21 JUN 1995 153534 -0400