Season the lamb shanks with salt and pepper and dust with the flour, shaking off any excess. In a large skillet, heat the olive oil until shimmering. Cook the lamb shanks over moderately high heat, in batches if necessary, until browned all over, about 10 minutes. Transfer the shanks to a slow cooker. Add the chicken broth, dill sprigs, garlic, onion and lemon. Cover and cook on high until the lamb is tender, about 4 hours.