In a small bowl, combine kosher salt, black pepper, garlic powder, onion powder, and dried thyme. Rub this spice mixture evenly over the entire beef roast.
Heat olive oil in a large skillet over medium-high heat. Sear the beef roast on all sides until browned, about 3-4 minutes per side.
Place the seared roast into the slow cooker.
In a separate bowl, whisk together beef broth, water, Worcestershire sauce, soy sauce, and Dijon mustard. Pour this mixture over the beef in the slow cooker.
Cover and cook on low for 8 hours, or until the beef is very tender and easily shredded.
About 15 minutes before serving, preheat the oven to 350°F (175°C).
Slice kummelweck rolls in half. Mix caraway seeds and kosher salt together and sprinkle generously on the cut sides of the rolls.
Place the rolls cut side up on a baking sheet. Optionally, spread butter on the rolls before sprinkling the caraway salt mixture for extra flavor.
Toast the rolls in the oven for 10-15 minutes until the tops are golden and crusty.
Remove the beef from the slow cooker and shred it using two forks. Keep the cooking liquid warm for dipping.
To assemble the sandwiches, pile shredded beef onto the bottom half of each 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 immediately with warm au jus from the slow cooker for dipping.