Step 1

Place shallot, vinegar, and salt in large bowl and toss to combine; set aside. Bring 2 quarts water to boil in large saucepan over high heat. Add green beans and ¼ cup salt, return to boil, and cook until green beans are bright green and tender, 5 to 8 minutes.

Step 2

While green beans cook, fill large bowl halfway with ice and water. Drain green beans in colander and immediately transfer to ice bath. When green beans are no longer warm to touch, drain in colander and dry thoroughly in salad spinner.

Step 3

Add green beans, oil, mustard, capers, tarragon, and pepper to bowl with shallot mixture and toss to combine. Transfer to platter and serve.