Quarter the onions and remove peels. Place half the onions on the bottom of the slow cooker.

Season both sides of the roast liberally with salt and pepper. Place roast on the bed of onions.

Press garlic cloves and put directly on roast.

Cover with remaining onion quarters.

Add 1 cup of balsamic vinegar and 1 cup of wine. Then pour in just enough beef broth to cover the roast. It will vary based on size and shape of the roast. I used 2 3/4 cups. Add the mustard, whisking it with a fork briefly to incorporate it. Finish by adding 2 bay leaves and a sprig of fresh thyme.

Cook covered on low for 6 hours or until you can pull the roast apart with a fork.

At the 6 hour mark, shred the roast with a fork, turn heat to high and let cook for another 30 minutes. Serve on rolls, topping with cheese if desired, or serve with a bed of mashed potatoes.