In a bowl, whisk the vinegar with the shallot and mustard. Whisk in the walnut and olive oils and season with salt and pepper.
In a medium bowl, toss the watercress with 3 tablespoons of the dressing. In another medium bowl, gently toss the crab with the celery, apple, walnuts and the remaining dressing. Season with salt and pepper. Transfer the watercress to plates, top with the crab salad and serve.