In a medium bowl, whisk vinegar and mustard until combined. Add olive oil in a slow stream, whisking constantly until emulsified. Whisk in salt and several grinds of pepper. Check for seasoning and add more salt or pepper to taste. Serve or refrigerate for up to 2 weeks.
Notes
If olive oil solidifies in refrigerator; remove 15-30 minutes before serving to warm and liquify. Whisk vinaigrette to combine before serving.