In a blender or food processor blend together mustard, vinegars, and salt and pepper. With motor running, add oil in a very thin stream and blend until emulsified. Vinaigrette may be made 4 days ahead and chilled in a tightly sealed jar. Shake vinaigrette well before serving. Arrange arugula, and tomatoes decoratively on a platter and serve with vinaigrette.
Recipe courtesy of Gourmet Magazine