In a 3-quart pot fitted with a steamer basket, steam the green beans for 7 minutes until they are bright green and crisp-tender (should have a little bit of a crunch). Using tongs, transfer green beans to a serving bowl.
In a food processor, pulse walnuts, olives, onions, and parsley on and off for about 30 seconds or until desired consistency is reached (should form a paste). Transfer to a small bowl and mix well with olive oil and vinegar. Toss tapenade with green beans and serve.