This is what I decided on for MIL's visit on Friday. I wanted something I could pop in the crockpot and would be ready regardless of what time she arrived (the trip can take 5 hours on a good day, 7 on a slow one).
4 lb. beef rump roast
1 cup water
1 cup vinegar
1 lemon, sliced with peel on
1 large onion, sliced
4 bay leaves
5 whole peppercorns
2 Tbsp. sea salt
2 Tbsp. raw sugar
15 gingersnaps, crumbled (does not need to be fine crumbs, just broken up)
Combine water, vinegar, lemon, onion, bay leaves, peppercorns, salt and sugar in deep ceramic or glass bowl (do not use metal). Add roast, cover and refrigerate 24-36 hours, turning meat several times (I turned mine twice a day, and it turned out great).
Put beef in crockpot and pour one cup of marinade (liquid only) over meat. Cover and cook on low 6 hours for a newer cooker, up to 8 hours for an older one.
Remove meat to a platter and cover with foil to keep warm. Turn crockpot to high and stir in gingersnaps. Cover and cook for 10- 15 minutes.
When sauce is ready, slice meat, and serve with sauce poured over top.
I served this with mashed potatoes (the sauce tasted great with them!) and green beans.
* Based on Old World Sauerbraten from Fix-It and Forget-It Lightly