In a large bowl, combine the watercress, cucumbers, peaches, and beef.
Pour the lime juice into a small bowl and whisk in the sugar and 1/2 teaspoon salt until dissolved. Add the garlic, ginger, and 1/4 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.
Tip: For cooked roast beef, look no further than the deli counter. Ask the server to forgo the slicer and cut a 1-pound slab by hand.