Heat the oil in a cast-iron skillet over high heat. Season the chuck roast with salt, pepper, and half of the rosemary. Sear for 2-4 minutes per side, until a browned crust forms (the roast will not be fully cooked).

Pour the broth into a slow cooker. Add the onion and remaining rosemary and stir to combine. Place the seared chuck roast on top.

Cover and cook on high for 4 hours or low for 8 hours, until the meat is tender.

Shred the beef with two forks and serve.