Salad: In a large pot of salted water, boil the red potatoes until just tender. Add the green beans to the potatoes, and cook for several minutes or until just tender. Drain potatoes and green beans into s large colander in sink and rinse with cold water. Let potatoes and green beans sit and drain briefly while preparing dressing.

 

Dressing: In a small bowl, whisk together mustard, lemon juice and olive oil. 

 

Scoop potatoes and green beans into a large bowl. Pour dressing over top and carefully toss until coated. Add salt and pepper to taste. Serve right away or refrigerate until ready to serve.