Heat olive oil in a skillet over medium-high heat. Brown the lamb shanks on all sides, then transfer to the slow cooker.

In the same skillet, add onion, garlic, carrots, and celery. Sauté until softened, then add to the slow cooker.

Pour beef broth over the lamb and vegetables. Add rosemary, thyme, parsley, salt, and pepper.

Cover and cook on low for 8 hours or until the lamb is tender and falling off the bone.

Once cooked, remove the lamb shanks and set aside. Strain the cooking liquid into a saucepan.

Mix cornstarch with water to create a slurry, then add to the strained liquid. Bring to a boil and stir until thickened.

Serve lamb shanks with the herb gravy poured over the top.