Coat the tri-tip roast with Stubb’s Beef Spice Rub on all sides and place in the slow cooker. Place the onion quarters around the beef roast. Pour broth, Beef Marinade, and water over the roast. Cook on high for 5 hours.
Remove the roast from the slow cooker and shred the beef, keeping the broth in the pot. Return the shredded beef to the broth and cook for an additional 1-2 hours on low.
Assemble sandwiches by filling the hoagie rolls with beef and topping with cheese slices. Serve with broth on the side for dipping.