1. Wipe roast with damp paper toweling. Sprinkle meat all over with salt, then place, fat-side up, on a rack in an open roasting pan. Stick meat thermometer into roast so the point reaches the center of the meat. Do not add water or cover pan. Place in oven.
2. Roast in slow oven (325°) for 1 hour and 20 minutes, or until thermometer registers 140° for rare or 160° for medium.
3. Remove meat from oven; place on serving platter; cover loosely with foil. Let stand about 20 minutes to make roast easier to slice.
4. Cut strings away from meat, then trim a thin slice from the bottom so roast will stand upright on platter. Place sprigs of watercress around the meat.