Add beef, jalapenos, onions and garlic to slow cooker. Secure lid and cook on low for 12 hours or high for 6 hours.
Remove lid from slow cooker. Remove bones (if any) and use tongs to shred meat. Recover with lid and keep warm until ready to serve.
In a sauce pan over medium heat, add butter, garlic and onion. Once butter is melted add flour and let cook for 3 minutes. Whisk in heavy cream and cumin. Cook for 3 minutes, then add jalapenos and Monterey Jack cheese. Stir until fully melted then add in broth and cheddar cheese. Continue to stir until all is incorporated. Reduce heat to medium-low and keep warm will you prepare the sandwiches.
For each sandwich - cut roll in half and place 2 slices of cheese on the bottom half. Load up with beef and place top of roll on, pressing down slightly to help hold together (you can also use a toothpick if you like). Cut sandwich in half and serve with a small dish of nacho cheese for dipping.