1/4 cup white-wine vinegar
2 tablespoons finely chopped shallot
1/2 teaspoon salt
1/2 teaspoon Dijon mustard
1/4 teaspoon black pepper
3/4 cup olive oil

Whisk together vinegar, shallot, salt, mustard, and pepper until salt is dissolved. Add oil in a slow stream, whisking until emulsified.

Cooks' note:
Dressing keeps, covered and chilled, 1 month.