Step 1

Use a vegetable peeler to shave the asparagus.

Step 2

For the dressing: Combine vinegar, shallot, mayonnaise, mustard, and salt in medium bowl and season with pepper to taste. Whisk until mixture is milky in appearance and no lumps of mayonnaise remain. Whisking constantly, very slowly drizzle oil into vinegar mixture until glossy and lightly thickened, with no pools of oil visible. (If pools of oil are visible on surface as you whisk, stop addition of oil and whisk until mixture is well combined, then resume whisking in oil in slow stream.)

Step 3

For the salad: Place lettuce, watercress, asparagus, prosciutto, and half of pine nuts in large bowl. Toss to combine. Drizzle with dressing and toss until greens are evenly coated. Season with salt to taste. Sprinkle with remaining pine nuts. Serve immediately.