Drain and rinse the cashews, and place in a blender. Add the nutritional yeast, lemon juice, mustard, black salt, turmeric, and cayenne, then pour in the boiling water. Blend on high speed for a minute, until smooth and creamy. Taste, adjust salt as needed, and serve immediately. To make ahead, store the finished sauce in the refrigerator, then reheat in a saucepan on the stove over low heat, stirring constantly.