In a large bowl, combine the watercress, cucumbers, peaches, and beef; set aside.
In a small bowl, combine the lime juice, sugar, and ½ teaspoon salt and whisk until dissolved. Add the garlic, ginger, and ¼ teaspoon pepper.
Slowly add the oil in a steady stream, whisking constantly until emulsified. Stir in the shallots.
Transfer the salad to a serving bowl or platter and drizzle the vinaigrette over the top.