Combine all the ingredients in a blender — or use an immersion blender — and turn the machine on; a creamy emulsion will form within 30 seconds.
Taste and add vinegar a teaspoon or two at a time until the balance tastes right to you.
If the mixture is too thick, add water a tablespoon at a time until the consistency is right.
Taste, adjust the seasoning, and serve.
This is best made fresh but will keep, refrigerated, for a few days; bring it back to room temperature and whisk briefly before using.