3 tablespoons white-wine vinegar
1 teaspoon Dijon mustard
1/2 teaspoon salt
1/8 teaspoon black pepper
2 teaspoons finely chopped shallot
6 tablespoons extra-virgin olive oil

Whisk together vinegar, mustard, salt, pepper, and shallot in a bowl, then add oil in a slow stream, whisking until emulsified.