In a large Dutch oven or oven-proof pot over medium-high heat, add the yellow onions, the carrots, the leek, the garlic, the thyme, the rosemary, the bay leaves, the juniper berries, the cloves, the black peppercorns, the kosher salt, the sugar, the red wine, the red wine vinegar, the beef broth, and the golden raisins and boil, about 10 minutes.
Let the marinade cool completely.
Nestle the roast beef into the marinade and cover tightly.
Marinate the roast beef mixture in the refrigerator, turning the roast beef over each day to ensure it marinates thoroughly, for at least 48 hours and up to 2 weeks.
Once you're ready to prepare it, pat the roast beef dry.
Pour the marinade into a large bowl.
Wipe out the Dutch oven with paper towels, place it over high heat, and heat the vegetable oil.
Add the roast beef to the oil and sear on all sides until browned, about 2 minutes per side.
Add the marinade to the roast beef and bring to a boil.
Cover the Dutch oven, reduce the heat, and simmer, until the roast beef reaches an internal temperature of 135 degrees F, about 2 hours-2 hours 30 minutes.
Let the roast beef rest for 10 minutes.
Pour the marinade through a large strainer sitting inside a large bowl.
Discard the solids and pour the liquid into the Dutch oven.
Add the crushed gingersnap cookies to the liquid and cook over medium-low heat, while stirring, until it thickens into a gravy, about 10 minutes.
Season the gravy with the salt and the black pepper.
Slice the sauerbraten into thin slices.
Serve the sauerbraten topped with the gravy.