Place the potatoes and carrots in the bottom of the slow cooker.
In a small bowl combine the Dijon mustard, rosemary, garlic cloves, thyme, and black pepper, Mix well.
Rub the Dijon mustard mixture all over the beef roast and place it in the slow cooker.
Sprinkle the chopped onions around the sides of the beef roast. Slowly pour the beef broth around the sides of the beef roast as well.
Cook for 4 hours on high or 7-8 hours on low.