Preheat oven to 425 degrees F.
Rub roast with olive oil, salt, and pepper. Place roast on rack in a shallow baking pan, tucking the thin end under to make it as thick as the rest of the roast.
Bake, uncovered, 40 to 45 minutes or until thermometer registers 130 to 135 degrees F. Remove from oven and transfer to a cutting board; let stand 15 minutes before carving. Reserve meat juice, and carve meat into very thin slices (as thin as you can slice).
Reduce oven temperature to 350 degrees F.
To assemble sandwiches, divide sliced beef on the bottom half of each roll, spoon with reserved beef juice, and top with the top half of each roll. Serve with horseradish on the side.
Makes 8 sandwiches.
In a small saucepan over medium-high heat, stir together water and cornstarch. Heat mixture to a gentle boil. Reduce heat to low, and stir until mixture thickens and is translucent. Remove from heat and let cool.
Beef on Weck Sandwich Recipe: https://whatscookingamerica.net/sandwich/beefonweck.htm