1. Bring a large saucepan of water to a boil over medium-high heat. Carefully add the green beans and return to a boil. Cook until the beans are crisp-tender, about 5 minutes. Drain and keep warm.
2. Melt the butter in a large skillet over medium heat. Add the vinegar and pepper. Stir in the walnuts and lemon zest and juice. Stir until heated through.
3. Place the green beans in a large serving bowl. Add the walnut mixture and toss gently. Serve immediately.
Variations
Substitute coarsely chopped pecans or pine nuts for the walnuts.
For added color, use half green beans and half yellow wax beans.