In a container with a tight lid, preferably a glass jar, combine shallot, vinegar and mustard.
Close tightly and shake well to mix.
Add salt and pepper, and shake again.
Add olive oil, 1/3 cup at a time, shaking very well after each addition, until smooth and emulsified.
Taste and add more olive oil, 1/4 cup at a time, if dressing is too tart.
Use immediately or refrigerate up to 3 weeks.
The chilled olive oil will form a lump, so remove from refrigerator 30 minutes before using to allow it to liquefy.
Shake well before serving to re-emulsify.
