- 3 lb yellow-fleshed potatoes such as Yukon Gold
- 1/4 cup cider vinegar
- 1 tablespoon grainy mustard
- 3/4 teaspoon salt
- 1/2 teaspoon black pepper
- 1/4 teaspoon sugar
- 1/2 cup thinly sliced shallots (2 medium)
- 1/2 cup olive oil
- 2 bunches watercress (4 cups loosely packed), coarse stems discarded
- Peel potatoes and cut into 3/4-inch pieces, then cook in a 3- to 4-quart saucepan of boiling salted water , uncovered, until just tender, 10 to 15 minutes. Drain in a colander.
- While potatoes are cooking, whisk together vinegar, mustard, salt, pepper, and sugar in a large heatproof bowl until combined, then stir in shallots and let stand until ready to use.
- Toss potatoes with vinegar mixture, then add oil and watercress, tossing gently to combine.