Beef Salad with Potatoes and Cornichons

Bake potatoes:

Put oven rack in middle position and preheat oven to 400°F.

Place potatoes on a 12-inch square of foil, then prick each with a fork and wrap in foil. Bake until tender, about 1 hour. (Potatoes are done when a small knife pierces centers easily.) Unwrap potatoes and cool slightly.

Assemble salad:

Reheat beef in a small baking dish, covered, in oven or in microwave until warm. While beef is heating, whisk together vinegar, mustard, shallots, sugar, salt, and pepper in a small bowl, then add oil in a slow stream, whisking until dressing is emulsified.

Steam beans in a steamer set over boiling water, covered, until just tender, about 5 minutes. Meanwhile, peel potatoes and cut crosswise into 1/4-inch-thick slices, transferring to a large bowl. Add hot beans to warm potatoes along with 2 tablespoons vinaigrette, gently tossing to coat.

Line a platter or 4 plates with lettuce leaves and arrange potato mixture on top. Toss meat, cornichons, and parsley with remaining vinaigrette and mound on top of potato mixture.