Melt the butter in a not-too-large pot. Flour the piece of meat and tap it gently to remove the excess flour. Quarter the onion.
Brown: Fry the piece of meat on all sides until nicely browned. Add the onion pieces after half of the frying time and fry them together with the meat, taking care that they don't get too dark; keep turning them around. When the meat is brown, add the vinegar and let it bubble away wholly.
Slow cook in oven: Sprinkle the meat with salt and pepper, add stock and cream. Put on the lid, turn the heat down to very low and simmer the meat for 3 hours.During the last hour of the cooking time, start preparing the side dishes.
Sauce: Take the meat out of the sauce and bind the sauce with 1 or 2 tablespoons cornstarch mixed with enough cold water to form a slowly runny paste. This step is optional; add cornstarch if you would like to have a thicker sauce.
Rest for about 10 minutes before slicing. Serve immediately.
The nutrition is only calculated for the meat and the sauce.