Bring a large pot of lightly salted water to boiling. Add mixed small red and fingerling potatoes. Cover and bring back to a boil, the 1 n uncover and cook for 20 minutes or until fork-tender. Drain. Cut warm potatoes into bite-size pieces and place in a bowl. Stir in thinly sliced red onion. In a small bowl, blend sherry wine vinegar, grainy mustard, salt and pepper. Drizzle in extra-virgin olive oil, whisking continuously. Stir in chopped fresh oregano and chopped fresh parsley. Pour dressing over potatoes and onions. Stir gently and cover. Refrigerate up to a day.