Preheat the oven to 350 F (180 C).In a saucepan, melt the butter and cook the onions gently until they are tender.Add the flour, mix well, and brown the roux a bit.Mix in the beef broth and let it thicken a bit.Add the vinegar and tomato paste, season to taste, and finish with parsley.Add the pot-au-feu slices and cook for 5 more minutes.Place in gratin dishes and top with some cheese.Bake for 30 minutes and serve immediately.