Season the roast with salt. Brown it in the oil for 4 minutes per side and place it in a 6-quart slow cooker.
Blend the tomatillos, green chiles, onion, garlic, broth, cumin and oregano until mostly smooth.
Arrange the potatoes around the beef and pour the green chile sauce over everything.
Cover and cook on low for 8 hours, until the roast pulls apart and the potatoes are tender. Transfer the beef and potatoes to a platter.
Skim the sauce, stir in the lime juice and taste for salt. Pull the beef into large pieces and spoon the green sauce over it.