In a large bowl, whisk together the oil, lemon zest, lemon juice, Dijon mustard, and 1/2 teaspoon each salt and pepper. Fill a large bowl with ice water. Bring a large pot of water to a boil. Add 1 tablespoon salt, then the green beans, and cook until just tender, 3 to 4 minutes. Drain. Transfer the green beans to the ice water to cool; drain and set aside. Toss the beans gently in the lemon vinaigrette.