In salad bowl, toss together carrots, celery and mint; set aside.
In skillet, heat oil over medium high heat.
Add onion, mustard seeds, salt and pepper; cook, stirring often, for 3 minutes or until onion is softened.
Stir in vinegar; immediately pour over salad and toss well.
Taste and adjust seasoning. (Salad can be covered and refrigerated for up to 2 hours.)