Whisk the vinegar, mustard, 1/4 teaspoon salt and a few grinds of pepper in a large bowl until combined. Slowly drizzle in the oil in a steady stream, whisking constantly, until the dressing is emulsified and smooth. (If you're using a vinegar high in acid, you might need to add a little more oil to taste.) Refrigerate any leftovers.
Photograph by Ralph Smith