Bring a large pot of lightly salted water to a boil. Add potatoes. Cover and bring back to a boil. Boil uncovered for 20 minutes or until fork tender. Drain.
Cut warm potatoes into bite-size pieces and place in bowl. Stir in onions.
In a small bowl, blend vinegar, mustard, salt and pepper. Drizzle in oil, whisking continuously. Stir in oregano and parsley. Pour dressing over warm potatoes and onions. Stir gently. Cover and refrigerate for up to a day.
Makes 8 servings.