Minted Green Beans with Red Onion

2 pounds green beans, trimmed
1 teaspoon Dijon-style mustard
1 tablespoon white-wine vinegar
1/4 cup olive oil
3 tablespoons minced fresh mint leaves
1/2 cup finely chopped red onion

In a kettle of boiling water cook the beans for 2 to 4 minutes, or until they are crisp-tender, transfer them with a slotted spoon to a bowl of ice and cold water to stop the cooking, and drain them well. Pat the beans dry with paper towels and chill them, covered, for at least 3 hours or overnight.

In a large bowl whisk together the mustard, the vinegar, and salt and pepper to taste, add the oil in a stream, whisking, and whisk the dressing until it is emulsified. Add the beans, the mint, and the onion and toss the mixture until it is combined well.

from: Gourmet Magazine