Boil well salted water in a large saucepan. Tie a string to that of the rump. Attach the string to the handle of a wooden spoon long enough to lay across the pan. When the water boils, place the pad of rump so that it is completely and freely bathed without touching any of the walls of the pan. For cooking, count precisely 23 minutes (the meat must sway without constraint as the water bubbles). Foam as you go.
Drain the meat and let it cool; pepper each side and cover with plastic wrap before putting it in the refrigerator.
Wash and wring the watercress, remove the thickest stems. Add the mayonnaise and add the finely chopped herbs and shallots, pickled gherkins and capers.
Season the watercress with 2 tablespoons of oil, 1 tablespoon of sherry vinegar, salt and pepper.
Take the meat out of the refrigerator. Mince in very thin slices and serve with mayonnaise and watercress salad.
Serve with Arbois wine.