Sauerbraten & Gingersnap Gravy

4 lb beef round tip
2 medium onions, thinly sliced
8 peppercorns, slightly crushed
4 whole cloves
1 bay leaf
1 C white vinegar
1/2 C cider vinegar
1 C water
2 C boiling water
10 gingersnaps, crushed
1/2 C plain nonfat yogurt

Marinate the beef in onions, peppercorns, cloves, bay leaf, vinegars, and water in the refrigerator for 1 to 2 days, turning twice each day. Remove the meat from the marinade and pat it dry. Strain the marinade; reserve onions and 1 C of liquid. Brown the meat on all sides. Add boiling water and gingersnaps; simmer, covered, for 1-1/2 hours, turning often. Add reserved onions and marinade; continue cooking until meat is tender, about 2 hours. Remove the meat and keep it warm. Strain the cooking liquid into a saucepan. Mix yogurt with flour and slowly stir into liquid, cooking over medium low heat and stirring constantly, until sauce is thick and smooth. Slice meat and add to the gravy.