Season the beef chuck roast evenly with salt and black pepper on all sides.
Heat the olive oil in a large skillet over medium-high heat until shimmering.
Sear the beef roast in the skillet for about 4 minutes on each side, until browned. Remove and set aside.
Place the sliced onion and minced garlic into the bottom of the slow cooker.
In a small bowl, whisk together the beef broth, tomato paste, Worcestershire sauce, Dijon mustard, dried thyme, and smoked paprika until combined.
Place the seared beef roast on top of the onions and garlic in the slow cooker.
Pour the broth mixture over the beef.
Cover and cook on low for 8 hours, or until the beef is very tender and easily shredded with a fork.
Remove the beef from the slow cooker and shred it using two forks.
Return the shredded beef to the slow cooker and stir to combine with the cooking juices.
To serve, pile the shredded beef onto the soft sandwich buns and spoon some of the cooking juices over the top.