Melt the butter in a Dutch oven over medium-high heat. Brown the beef on all sides, working in batches so as not to overcrowd. Transfer to a plate and set aside. Leave the browned bits in the pan (important for flavor).Add the onions and some more butter if needed and cook until caramelized, about 25 minutes. Add the flour and stir until combined. Add the beef to the onions, stir to combine, and cover with the beef stock just until covered. Add the seasonings and red wine vinegar, stir to combine.Increase the heat and bring the stew to a boil. Reduce the heat to low, cover, and simmer for 2 1/2 hours. Uncover and simmer for another 30 minutes to further thicken the stew. Add salt, pepper and more red wine vinegar to taste. Discard the juniper berries, cloves, and bay leaves. Leave in or discard the peppercorns.Serve with mashed potatoes and braised red cabbage.