I used to like the Jolly Green Giant canned stuff until I realized how easy it is to make Three Bean Salad yourself. I pared my version down to the key elements in order to minimize fuss (it comes together in about 15 minutes), but you can easily embellish it: use fresh green beans or a mixture of green and yellow beans; add a spice like curry or chile powder; or use a mixture of fresh herbs, like dill, mint and thyme, along with the parsley. The salad can be made ahead, with the dressing acting as a marinade (add the parsley just before serving). However, the longer it sits, the duller the color of the green beans. A few hours is optimal, but I think a day ahead would be fine too. —julie_chicago
In a large bowl, combine beans and onion. Prepare an ice bath (fill a large bowl halfway with ice and add cold water). Steam green beans in microwave according to package directions (I like the beans slightly under done and firm for this salad). Immediately chill green beans in ice bath for about 3 minutes to stop cooking and preserve bright color. Drain and air dry or pat dry with paper towel. Add to bowl with other beans.
In a small jar with a tight-fitting lid, combine oil, vinegar, sugar and mustard. Shake until emulsified (if you don't have a jar, whisk in a bowl). Drizzle about two-thirds of dressing over bean mixture and toss. Add more dressing if desired. Season to taste with salt and pepper. Just before serving, add parsley and toss again.