The marinade - 48 to 72 hours in advance: Place the peppercorns, juniper and allspice into a mortar or small plastic bag, or fold up a piece of waxed paper. Crush with the pestle if using the mortar or a rolling pin or hammer if using the bag or paper. Transfer the crushed spices to a small saucepan. Add the bay leaves, cloves, salt, vinegar, wine, and onion slices and bring to a boil. Transfer the marinade to a large bowl and let cool to room temperature. (I prefer a glass or stainless steel bowl. Whatever you use, don't use plastic: the acidic marinade will absorb flavors from plastic.)
Tying and marinating the meat: Using kitchen string, tie the beef into a neat, compact shape. As soon as the marinade is cool, add the beef to the bowl and roll in the marinade to coat all sides. Cover, refrigerate and marinate for 2 to 3 days, turning the meat once or twice a day. (There is no special timing here for turning the beef in the marinade. You just want to make sure that over the course of the 48 or 72 hours, the meat is turned 3 or 4 times so it marinates evenly.) Heat the oven to 300 degrees F.
Browning the meat: With tongs, lift the meat out of the marinade, scraping any onion slices or spices that stick to the meat back into the marinade, and transfer it to a plate. Reserve the marinade. Pat the meat thoroughly dry all over with paper towels. Heat the butter and oil in a large Dutch oven or other braising pot over medium heat. Add the meat to the pot and brown well on all sides, 20 to 25 minutes total. (You may need 2 implements to turn the meat: try tongs and a large metal spatula. Lift the meat from the pan with the spatula, grab with the tongs, and turn.) Transfer the meat back to the plate. Pour off all the fat from the pan, and deglaze the pan with the reserved marinade, scraping any browned bits to loosen. Bring the marinade to a simmer and add the meat. Cover with parchment paper, pressing down so it nearly touches the meat and the edges of the paper hang over the sides of the pot by about 1-inch. Set the lid firmly in place, and transfer to the lower third of the oven.
The braise: Braise the meat at a gentle simmer for 1 1/2 hours. Check after about the first 10 minutes to see that the liquid is not simmering too energetically; if it is, lower the oven by 10 to 15 degrees. After 1 1/2 hours, turn the meat over, using the tongs, and metal spatula, and continue braising gently for another 1 1/2 hours, or until fork-tender.
The finish: With the tongs and metal spatula, transfer the meat to a cutting board with a moat and cover loosely with foil to keep warm. Strain the cooking juices into a saucepan and let sit for a minute. Gently tilt the pan and skim off the fat with a large spoon. Whisk in the gingersnap crumbs and sugar, place the pan over medium-high heat, and bring to a boil. Gently boil the sauce for 5 minutes, whisking often, to reduce and thicken it slightly. Lower the heat to low and whisk in the sour cream until smooth. Heat through, but do not let the sauce boil, or the sour cream will curdle. Taste the sauce for salt.
Serving: Remove the strings from the meat, and pour any accumulated juices into the sauce. Carve the beef into thick slices. If the slices crumble, which they sometimes will, just cut into irregular pieces and arrange on a platter. Spoon the sauce over the beef and serve.