In a large pot of lightly salted boiling water, cook the beans until crisp-tender, about 3 minutes. Drain and run under cold water to stop the cooking.
Meanwhile, toast the walnuts in a skillet over medium heat until lightly golden, 6 to 7 minutes, stirring occasionally. Reserve half the walnuts and crush the remainder in a resealable plastic bag with a meat pounder or rolling pin. In a large bowl, whisk together the oil, lemon juice, salt, and a few grinds of pepper. Add the crushed walnuts, whole walnuts, and beans and toss to coat well. Serve cold or at room temperature. (Can be made up to 1 day ahead. Cover and refrigerate.)