How to Make It
Peel carrots, potatoes, onion, and turnip and cut into about 1/2-inch dice.
Rinse ribs, drain, and lay bone-down in a single layer in a 12- by 14-inch casserole (about 5 qt.) at least 2 inches deep. Distribute vegetables evenly around but not over meat. Pour broth and 1 1/4 cups water into casserole.
Coat tops of ribs with 6 tablespoons horseradish.
Cover casserole tightly with foil, tenting if necessary to keep foil from touching ribs. Bake in a 375º oven until meat is very tender when pierced, about 3 hours.
Meanwhile, break leaves from watercress and finely chop stems. Cover each separately and chill.
Uncover casserole and bake until ribs brown, about 10 minutes. With a slotted spoon, transfer ribs to a large platter and keep warm. Mix chopped watercress stems with vegetables and return to oven until watercress is slightly wilted, about 5 minutes, stirring once or twice. With slotted spoon, transfer vegetables to platter. Skim and discard fat from pan juices, then pour juices into a bowl.
Garnish platter with the watercress leaves. Serve the meat and vegetables with pan juices, horseradish, and salt to taste.