Cook potatoes in pot of lightly salted boiling water for 15 minutes or until tender. Add the green beans for the last 5 minutes of cooking.
Meanwhile, saute onion in oil in medium-size skillet over medium heat for 10 minutes or until softened. Remove from heat; whisk in mustard, vinegar, sugar, salt and pepper. Drain potatoes and green beans; toss in serving bowl with mustard vinaigrette. Serve warm. Makes 8 servings.