Vinaigrette: In a mixing bowl, whisk mustard, vinegar, and crème fraîche to combine. Add the buttermilk and olive oil and whisk again to emulsify. (If the vinaigrette breaks—i.e., doesn’t emulsify—whisk in ½ to 1 teaspoon water.) Finally, add curry powder, season to taste with salt, and whisk to combine. Makes about 1 cup and can be stored in the refrigerator up to 1 week.
Assembly: Put watercress and arugula in a serving bowl; gently toss with ¼ cup dressing (or more if desired). Garnish with sliced avocado, crushed cashew nuts, and a sprinkle of salt before serving.