In a small bowl, combine kosher salt and black pepper with garlic powder and onion powder. Rub this seasoning mixture evenly over the beef roast.
Heat olive oil in a large skillet over medium-high heat. Sear the roast on all sides until browned, about 3-4 minutes per side.
Place the seared roast into the slow cooker.
In a separate bowl, mix beef broth, Worcestershire sauce, and Dijon mustard. Pour this mixture over the roast in the slow cooker.
Cover and cook on low for 8 hours, or until the beef is tender and shreds easily.
While the beef cooks, prepare the kummelweck topping by mixing kosher salt and caraway seeds on a small plate.
Slice the kummelweck rolls in half and lightly toast them. Immediately after toasting, press the cut sides into the salt and caraway seed mixture to coat.
Once the beef is done, remove it from the slow cooker and shred it using two forks.
To assemble each sandwich, pile shredded beef onto the bottom half of a kummelweck roll. Add a generous spoonful of prepared horseradish on top of the beef.
Place the top half of the roll on the sandwich and serve with warm au jus from the slow cooker for dipping.