In slow cooker, mix Au Jus mix according to package directions. Stir in garlic, salt and pepper. Add shredded roast beef. The beef should be covered with Au Jus, if not add enough beef broth to cover. Cook in slow cooker on low for 3-4 hours.
Slice pepper and onions. Saute in one Tablespoon of butter.
Butter buns with remaining butter. Broil buns about 1 minutes, or until golden brown.
Slice cheese. Layer cheese on one side of each browned hoagie buns. Return to oven, on lower rack, until cheese is melted.
With a slotted spoon remove meat from slow cooker and pile it on the melted cheese.
Add onions and peppers to the meat.
Serve with Au Jus.
NOTE: I usually prepare this meal in advance. See blog post for suggestions.