In a medium bowl, whisk together soy sauce, beef broth, Worcestershire sauce, olive oil, Dijon mustard, brown sugar, minced garlic, dried thyme, dried rosemary, black pepper, and salt to create the marinade.
Place the beef chuck roast in a large resealable plastic bag or shallow dish and pour the marinade over the roast. Seal or cover and refrigerate for at least 4 hours or overnight for best flavor.
Remove the roast from the marinade, reserving the marinade for cooking.
Place the sliced onion, carrots, and celery in the bottom of the slow cooker.
Put the marinated roast on top of the vegetables in the slow cooker.
Pour the reserved marinade evenly over the roast and vegetables.
Cover and cook on low for 8 hours, or until the beef is tender and easily shredded with a fork.
Once cooked, remove the roast from the slow cooker and let it rest for 10 minutes before slicing.
Serve the sliced roast beef with the cooked vegetables and spoon some of the cooking juices over the top.