Place roast in a slow cooker; pour reserved pepper juice over top. Cover and cook on low setting for 6 to 8 hours, until very tender. Remove roast and shred with 2 forks; stir back into liquid in slow cooker. Add peppers and onion.
Blend au jus mix with a little of the liquid from slow cooker until dissolved. Pour over beef; add water if needed to cover roast.
Cover and cook for an additional hour. Serve beef spooned onto rolls.