The mustard dressing and basil leaves perk up this potato-and-green-bean salad.
Bring a large pot of water to a boil. Add 1 teaspoon salt and potatoes. Return to a boil, and cook until potatoes are almost tender, 6 to 7 minutes. Add green beans; cook until potatoes are just tender and beans are bright green and tender, about 4 minutes. Drain. Let cool slightly.
Whisk together lemon juice, mustard, and 1 teaspoon salt in a large bowl; season with pepper. Add oil in a slow, steady stream, whisking until emulsified. Add potatoes and green beans; toss to combine. Add basil; toss. Serve salad warm or at room temperature.
You can cook the potatoes and green beans the day before and refrigerate them in an airtight container. Bring to room temperature, then toss with the dressing and basil just before serving.