1 cup unfiltered apple cider
3 tablespoons finely chopped shallots
2 tablespoons cider vinegar
2 teaspoons grainy mustard
1/3 cup olive oil
Boil cider in a small heavy saucepan until reduced to about 1/3 cup, 12 to 15 minutes.
Whisk together reduced cider, shallots, vinegar, and mustard with 1/4 teaspoon each of salt and pepper. Add oil in a slow stream, whisking until combined.