Preheat the oven to broil. Cut the bread loaf in half horizontally, place the halves on a baking sheet cut side up, and toast them in the oven until light brown, 1 to 2 minutes.
Spread the horseradish on the bottom half of the loaf, then layer on the roast beef.
To make the jus, mix the stock and Worcestershire sauce in small bowl. Baste the top half of bread with 1/4 cup of the jus. Place top half of the bread over the roast beef to complete sandwich. Cut the sandwich in quarters and serve with the remaining jus for dipping.
