Place roast in slow cooker and spread mustard over top of roast.
Pour onion soup mix over top of mustard.
Cover and cook for 8-10 hours on low.
Remove from slow cooker and allow to sit for 5 minutes. While the meat is resting, strain the broth to use for au jus.
Butter each side of the hoagie rolls and toast until barely brown.
Slice roast and place on each sandwich. Serve with au jus for dipping.
Because the crockpot traps moisture inside the pot and cooks slowly, this recipe is suitable for less tender (and thus less expensive) cuts of meat. If you want to ensure lots of au jus, you can add one half to a full can of beef broth—put the broth in the crockpot first, then set the roast gently in the broth and add the mustard and soup mix as directed.Dried mustard can be substituted for the prepared mustard, but use it in a ratio of 1 teaspoon dry mustard to 1 tablespoon prepared mustard. Mix it with a couple of tablespoons of water before adding to the roast.