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.