Combine garlic, rosemary, lemon zest, salt and pepper; rub all over lamb. Place in slow cooker. Whisk lemon juice with honey; pour over lamb. Add shallots. Cover and cook on low until lamb is tender, 7 to 8 hours.
Remove lamb to cutting board; tent with foil and let stand for 10 minutes before slicing. Skim fat from cooking liquid.
Whisk cornstarch with 2 tsp water; whisk into slow cooker. Cover and cook on high until thickened, about 10 minutes. Serve with lamb.
