Season roast with salt and pepper and place in a large glass or ceramic dish. Add cider vinegar, water, and sauerbraten spice. Cover and refrigerate 2-3 days, turning meat twice daily. The longer it marinates, the more sour the meat will be.
Remove meat from marinade and brown in bacon fat in a large pot. Strain the marinade and add 1 cup to the meat. Simmer, covered, 2 - 3 hours until tender. Add more marinade during cooking if necessary. Remove meat from liquid and let stand 15 minutes before slicing.
While meat is resting, add sugar and crumbled gingersnaps to the liquid in the pot. Stir until gravy thickens, adding a little flour or arrowroot if desired. Pour gravy over sliced meat to serve.