In a bowl, combine the sliced onion with ice water to cover and let soak for 30 to 45 minutes. Meanwhile, make the vinaigrette.
In a glass bowl, whisk together the vinegar, mustard, pepper, salt, and cayenne until thoroughly blended. Drizzle the olive oil over in a thin stream, whisking constantly until all the oil is incorporated.
Drain the soaking onions, pat them dry with paper towels, and separate into rings. In a large bowl, toss the tangerines and the watercress together gently. Drizzle over enough vinaigrette just to coat and toss again. Mound some salad onto the plate and scatter a few onion rings over the top.
c.1997, M.S. Milliken & S. Feniger, all rights reserved