Roast beef and au jus: preheat oven to 450 degrees F (232 C).
Slice garlic cloves into thin slices.
Cover beef all over with salt and pepper. Cut a dime-sized slit into the top of the beef and insert a piece of sliced garlic clove. Continue slicing the beef and inserting garlic into the top evenly all over the exposed surface of the beef. Add the beef garlic-side-up to a roasting pan with 2 cups of beef broth in the bottom of the pan.
Add the roasting pan full of beef to the oven and cook for 10 minutes at 450 F.
After 10 minutes, reduce the heat to 250 F (120C) and continue cooking the beef until the internal temperature is between 130 and 135. This will take around an hour, but it's best to try to cook the roast beef to temperature instead of time.
Once the internal temperature is reached, let the roast rest for 30 minutes before slicing.
Pour the au jus in the bottom of the pan into a sealed container for warming up later. Make sure to scrape up any beef bits on the bottom of the pan into the au jus.
After the beef has cooled, slice as thin as possible with a sharp knife or meat slicer.
Kummelweck roll: Preheat your oven to 350 degrees F (175 C).
In a small pot over medium-high heat bring 1 cup of water and 1 tablespoon of cornstarch to a boil.
After 5 or 10 minutes the cornstarchy water should have thickened. Remove the pot from the water and allow it to cool for 5 minutes.
Paint water and cornstarch with a pastry brush or the back of a spoon on top of a bun and immediately sprinkle it with caraway seeds and salt. Bake the bun for 5 minutes which will allow the cornstarch and water to glue the seeds on top of the bun.
Sandwich assembly: slice and toast your kummelweck roll if desired.
In a small pot add 1/4 cup of au jus and bring to a simmer. You could also do this in the microwave in a small bowl.
Dip the sliced part of your top bun in warm au jus.
Add a few slices of beef to the bottom of the roll. Top the beef with a tablespoon or two of prepared horseradish. Spread the horseradish around as evenly as possible.
Top the sandwich with the au jus dipped bun.
Serve with the warm au jus in a small bowl for dipping if desired.