This can definitely be made ahead, with the dressing acting as a nice 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.
Serves 4 to 6
1 (15 oz) can red kidney beans, rinsed and drained
1 (15 oz) can chickpeas, rinsed and drained
1/2 medium red onion (halved lengthwise), sliced paper thin
1 (12 to 16 oz) bag frozen green beans
3 Tbs safflower or canola oil
3 Tbs white wine vinegar
2 Tbs granulated sugar
1 teaspoon Dijon mustard
Salt and freshly ground black pepper
1/2 cup chopped parsley
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.