In a large bowl, mix vinegar, water, onions, sugar, bay leaves, peppercorns, cloves, and juniper berries.
Place the beef roast in the marinade, cover, and refrigerate for at least 2 days, turning occasionally.
Remove the meat from the marinade and pat dry. Strain the marinade and set aside.
Heat oil in a pot and sear the beef on all sides until browned.
Pour in the strained marinade and beef broth, then bring to a simmer.
Cover and cook on low heat for 2–3 hours until the meat is tender.
Remove the beef and set aside. Strain the sauce, then whisk in flour and butter to thicken.
Slice the beef and serve with the sauce.
