You can make French dip sandwiches several ways, but we chose the slow cooker for this recipe because the heavy lifting of tenderizing the beef and onions is left up to the low and slow heat of the appliance, not your work over a stove. For busy nights and weekends, that's precisely the type of recipe we need. These slow-cooker French dip sandwiches feature tender beef sandwiched between provolone cheese and toasted hoagie rolls. The meat is so tender you won't even really need to slice…