Season the roast with salt and pepper to taste, place it in the slow cooker, top with the onions, garlic, rosemary, thyme, and broth and cook on low for 8-10 hours.
Remove the roast from the slow cooker and set aside.
Skim 4 tablespoons of the fat from the juices in the slow cooker and heat it in a saucepan over medium heat, sprinkle in the flour and cook until lightly golden brown, add the remaining liquid from the slow cooker along with the mustard, Worcestershire sauce and pepper and simmer until it thickens, about 2 minutes before seasoning with salt to taste.
Slice the beef and serve topped with the gravy.
Option: Brown the beef on all sides before adding to the slow cooker.
Option: Saute the onions and garlic before adding to the slow cooker.
Tip: Cook for about 7-8 hours on low for a roast that is somewhat slice-able or 9-10 hours for a roast that is just falling apart tender!
