In a small bowl mix together the honey, Dijon mustard, rosemary, black pepper, lemon zest, and garlic.
Place the leg of lamb in a large plastic container or bowl and using your hands rub the spice mixture over the lamb. Cover with plastic wrap to seal the spices in and refrigerate overnight.
Place the lamb in the slow cooker, add salt to taste, and cook on low for 7-8 hours or to the desired doneness.