Step 1Measure the depth of a large pot by tying a length of string around the roast and then suspending the roast inside the pot so that the roast doesn't touch the bottom. Lay a wooden spoon across the top of the pot like a bridge. Tie the string to the spoon. With the meat still suspended, fill the pot with enough water to submerge the roast.
Step 2Remove the meat and the spoon, cover the pot tightly with a lid and bring the water to a rolling boil over high heat. Add 2 tablespoons of salt and carefully lower the beef into the water so it hangs from the spoon, not touching the bottom. Prop the lid on top until the water returns to the boil, then remove the lid. Cook the beef 15 minutes per pound for a center that is on the cool side of medium rare, 20 minutes per pound for medium.
Step 3When the meat is done, remove it and the spoon from the boiling water, cut loose the suspending string, cover the roast loosely with foil and set aside for 10 to 15 minutes.
Step 4While the beef is resting, trim the tough stems from the watercress and place it in a salad bowl. In a small mixing bowl, beat the vinegar into the mustard, then gradually add the oil, beating with a fork to make a smooth dressing. Season to taste with a little salt. Pour it over the watercress and toss well.
Step 5Cut free the strings that bind the roast and carve the meat into 1/4-inch slices, being careful to reserve all meat juices. Arrange the slices in the center of a warm oval platter. Arrange the watercress salad around the outside of the roast, and then the bread slices on top of that.
Step 6Serve immediately, passing the very coarse salt at the same time to sprinkle on the meat according to taste. Use the bread to sop up as much of the meat juice as possible.