Generously season the flank steak on both sides with salt. Place it in the bottom of a 6-quart slow cooker.
Add the chipotle peppers with adobo sauce and beef broth to the slow cooker.
In a small bowl, mix the cornstarch with 1 tablespoon water until smooth. Whisk this mixture into the slow cooker.
Cover and cook on low for 7 hours, or on high for 4 hours.
Remove the flank steak from the slow cooker. Return the lid to keep the sauce warm.
Using two forks, shred the flank steak and return the meat to the slow cooker. Mix to combine the shredded meat with the sauce.
Taste and adjust salt if necessary before serving.
Enjoy the spicy, tender beef in tacos, with rice, or alongside your favorite potato dish!