Place potatoes in a medium saucepan and cover with lightly salted water by 1 inch. Bring to a boil over medium heat and cook until tender, about 15 minutes.
Meanwhile, whisk shallots, vinegar, mustard, parsley, tarragon and water in a small bowl. Slowly whisk in oil. Season with salt and pepper.
Drain the potatoes and rinse with cold water. Divide lettuce among 4 plates; arrange the potatoes, tomatoes and beef on top. Drizzle with the dressing and serve.