In a medium bowl, whisk together the mustard, vinegar, salt, and pepper. Gradually add the oil and whisk until well combined; set aside. Bring a large saucepan of salted water to a boil. Add the green beans and cook until just tender, 6 to 8 minutes. Drain, run under cold water to cool, and set aside until you're ready to assemble the salad. In a large bowl, combine the green beans, fennel, and walnuts. Add the goat cheese and vinaigrette just before serving. Toss well and serve at room temperature or chilled.