Slice beef as thinly as possible. Set aside. In a large skillet, melt the butter and lightly sauté the onions. When golden and very soft, about 10 minutes, sprinkle them with enough flour to make a light roux.
Stir the mixture thoroughly for 2 – 3 minutes before adding the vinegar and mustard. Continue stirring, adding enough stock or Pot-au-Feu broth to make a thin sauce. Add the spices. Bring the sauce to a boil, then lower heat and simmer for about 30 minutes.
Assembly: Pour a layer of sauce into a shallow baking dish. Layer meat slices, always overlapping, alternating with more of the sauce. You should end up with a final layer of sauce. Bake, covered, in a 350° F oven for half an hour, if meat was originally poached, or for an hour, if roast beef.
Check to make sure sauce does not reduce too much. Add more stock if necessary. When preliminary cooking is finished, sprinkle surface of dish with a thick coating of fresh bread crumbs. Spoon just enough butter over crumbs to moisten them.
Return to oven for about 30 more minutes, or until a golden crust forms. Sprinkle the top with the parsley to garnish, and squeeze over it the juice of half a lemon. Serve at once. Variations: Add tomatoes, garlic, and/or mushrooms.
Instead of vinegar, use a little grated horseradish. In place of some of the stock, use wine.Servings: 2 to 4