In a mason jar, combine shallot, vinegar and mustard. Close tightly and shake well to blend. Add lemon juice, salt and pepper, and shake again.
Add olive oil 1/4 cup at a time, shaking very well after each addition, until the vinaigrette is well blended 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. Before using, shake well.