Trim excess fat from the roast; cut the roast in half. Place in a large bowl or nonreactive container.
Combine the sliced onion, vinegars, water, salt, sugar, cloves, peppercorns, bay leaves, mustard seeds, and juniper berries, if using.
Pour the mixture over the roast. Cover and refrigerate for 24 hours, turning occasionally.
Take the roast from the marinade; reserve 1 1/2 cups of the marinade and discard the rest.
Put the roast in a 5- to 7-quart slow cooker; pour the 1 1/2 cups of reserved marinade over the roast. Cover and cook on HIGH for 1 hour. Reduce the heat to LOW and continue cooking for about 7 hours, or until tender.
Remove the meat to a serving dish and keep warm.
With a sieve over a bowl, strain the juices. Discard the solids and pour the strained juices into the slow cooker. Add the crumbled gingersnaps. Cover and cook on HIGH for 15 minutes. Serve the sauce with the roast.