In a small bowl, combine kosher salt and caraway seeds to prepare the kummelweck topping.
Preheat a large skillet over medium-high heat and add olive oil.
Season the beef chuck roast evenly with salt, black pepper, garlic powder, and onion powder.
Sear the beef roast in the skillet for about 3-4 minutes per side until browned on all sides.
Transfer the seared beef roast to the slow cooker.
In a bowl, whisk together beef broth, Worcestershire sauce, and Dijon mustard.
Pour the broth 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 30 minutes before serving, prepare the kummelweck rolls: brush the tops with melted butter and sprinkle generously with the kosher salt and caraway seed mixture.
Place the rolls on a baking sheet and toast in a 350°F (175°C) oven for 10-12 minutes until golden and fragrant.
When the beef is done, remove it from the slow cooker and shred it using two forks.
Return the shredded beef to the slow cooker and stir to coat with the cooking juices.
To assemble each sandwich, slice a kummelweck roll in half horizontally, spread a generous amount of prepared horseradish on the bottom half, pile on the shredded beef, and top with the other half of the roll.
Serve immediately with warm au jus from the slow cooker for dipping.