Start this recipe 2 or 3 days before you plan to serve it. Lard the beef with little strips of bacon.
Peel knob celery, parsnips and carrots; cut into small pieces. Mix 3cups water, the vinegar and wine in a large roasting pan (not aluminum). Add the vegetables and bring to a boil.
Put in the beef, grated lemon rind, peppercorns and bay leaves. Put in the refrigerator to marinate for 2 or 3 days.
When you are ready to cook, wilt the onions in 1teaspoon lard over very low heat; do not let the onions brown.
Remove beef from marinade and place it on top of onion. Cook meat for 2 or 3 minutes on each side.
Pour marinade over meat and cook, covered, over low heat until meat is almost cooked, for 2 hours or more.
Take out meat and cut into ½ -inch-thick slices.
Use the 2tablespoons lard and the flour to make a roux; when properly golden brown, pour in ½cup cold water and whip until smooth.
Force vegetables and broth in which meat cooked through a sieve, or purée in a blender, and pour into the diluted roux.
Brown the sugar in a small frying pan until caramelized. Add 3tablespoons cold water and cook for a few minutes till sugar melts. Whip in the mustard. Pour the caramel and mustard into the puréed sauce.
Add sliced meat; try to keep the slices whole. Cook at a slow simmer for another 10 to 15 minutes to allow sauce to permeate the meat.