Pound the meat well. Melt lard in a large frying pan. When lard is very hot, brown beef on both sides over high heat for 1 or 2 minutes. Remove meat and put it aside.
Take all the onion and half of the other vegetables, and cut them into thin slices. Fry them in the hot fat for a few minutes.
Add 2cups water, the bay leaf, salt and pepper. Grate the lemon rind of the ½ lemon and add the rind to the vegetables. Squeeze the juice of the lemon and set it aside.
Return beef slices to pan and cook it all over very low heat until meat is done.
Cut the rest of the vegetables into long matchstick shapes.
Melt butter in another pot and cook matchstick vegetables, covered, adding very little water when needed.
When meat is done, remove it from the pan. Purée the broth and its vegetables in a blender, or force through a sieve. Add mustard, the lemon juice, and the flour mixed with 3tablespoons water. Finally, add the sour cream.
Put meat back into the puréed sauce and cook it for a few minutes.
Place meat slices neatly on a serving platter. Pour sauce over, then top with the matchstick-cut vegetables. Sprinkle with the chopped parsley.