Put the oil, lemon juice, a pinch of salt, and lots of pepper in a blender and turn the machine on; a creamy emulsion will form within 30 seconds. (A tablespoon of warm water will help the mixture emulsify.) Taste and add more vinegar a teaspoon or 2 at a time until the balance tastes right to you.
Add the shallot if you’re using it, and turn the machine on and off a few times until the shallot is minced within the dressing. 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.)