Place kale in a large bowl.
Add half the apple to kale along with celery, ¼ cup walnuts and ¼ cup raisins. Put remaining apple in a blender along with remaining ¼ cup walnuts, remaining 2 tablespoons raisins, mustard, 2 tablespoons water, vinegar and salt.
Purée until well combined and slightly thick, adding water if needed to thin. Pour dressing over kale salad and toss to combine
