Let the shallot and ginger macerate in the vinegar while you go about preparing the salad ingredients. Add a pinch of salt.
Chop the watercress coarsely, stems and all. Peel and cube the avocado.
Combine the oil with the vinegar, shallot, and ginger mixture. Taste for salt. Toss the watercress, mango, and cucumber together with the dressing, leaving the av