To make the dressing, in a small bowl, whisk together the vinegar, lemon juice, and mustard until blended. Whisk in the oil, shallot, and salt and pepper to taste.
Working with 1 orange at a time, cut a slice off the top and bottom to reveal the flesh. Stand the fruit upright. Using a sharp knife, thickly slice away the peel and pith, cutting downward and following the natural contour o