Bring a large pot of water with a steamer basket to a boil, add green beans and steam for about 4 minutes. Transfer to a serving bowl.
Toast the walnuts in a small dry skillet over medium heat until they become fragrant, about 2 minutes, and then transfer them to a small bowl to cool. Add the parsley and onion to the walnuts and stir to combine.
In another small bowl, whisk together the oil, vinegar and mustard. Toss the dressing with the green beans, top with the walnut mixture and season with salt and pepper. Serve warm or at room temperature.