Season chuck roast with salt and pepper.
Heat a large pan or dutch oven to a high heat.
Add oil and then the chuck roast. Cook for 2-3 minutes without moving the roast until browned.
Flip the chuck roast over and brown the other side for 2-3 minutes.
Remove the roast from the pan and rub with mustard.
Place the roast in the slow cooker on top of half of the sliced onions and garlic. Cover the roast with the remaining onions and garlic.
Without washing the mustard off of the roast, pour in the beef stock, red wine, and Worcestershire sauce.
Cook on high for 4-5 hours or low for 8-10 hours until the meat easily separates, and is fork tender. Taste au jus to check if additional salt and pepper is desired.
Toast lightly buttered buns, baguettes, or French bread topped with cheese of choice in the oven until the bread is warm and the cheese is melty.
Pile buns high with shredded beef and onions. Serve with au jus along side for dipping.