Put the oil, vinegar, and mustard in a small glass jar along with a pinch each of salt and pepper.
Screw on the top of the jar and shake until the dressing becomes thick and creamy. Taste and adjust the seasoning, adding small amounts. Add more vinegar 1 teaspoon at a time until you reach your desired balance.
Shake again and serve right away or store for up to 3 days in the refrigerator. Shake well before each use.
Notes:
Variations:
1 minced garlic clove or finely chopped shallot
1/4 cup chopped fresh parsley, basil, or dill or 1 tablespoon rosemary, tarragon, or thyme leaves