Sweet-and-Sour Cucumber Salad

In a medium bowl, toss together cucumbers and onions. Set aside.

In a small bowl, whisk together sugar, salt, mustard, and celery seeds. Whisk in oil and vinegar until emulsified. Pour vinaigrette over vegetables; toss to coat. Cover, and refrigerate at least 4 hours to let flavors blend. Serve chilled.