Chop the onions and carrots. Place them in a saucepan with the beef, peppercorns, juniper berries, salt, and bay leaves. Pour in roughly enough vinegar to completely cover the meat. Cover and refrigerate for 3 to 4 days.
Remove the beef from the marinade. Transfer the beef to an ovenproof saucepan with a lid (or baking dish covered with aluminum foil), along with 1/4 liter of the marinade and the vegetables.
Cook in the oven at 200°C / 392°F (using top, bottom, or convection setting) for 2 to 2.5 hours until the beef is tender. Remove the beef. Strain the remaining marinade and thicken it with sauce thickener.