Fill a large bowl with ice and water; set aside. Bring a pot of water to a boil. Salt the water; add the string beans, and cook until bright green and just tender, 3 to 4 minutes. Drain, and transfer to ice bath until cool. Drain, and cut in half lengthwise.
Peel cucumbers, and split lengthwise. Remove seeds using a melon baller or a spoon. Cut into 1/2-inch-thick slices on the diagonal. Combine with string beans, olives, and parsley leaves in a medium serving bowl.
Place mustard, red-wine vinegar, and salt and pepper to taste in a small bowl; whisk to combine. Slowly add olive oil, whisking constantly until well combined. Toss with salad just before serving.