Bring a pot of salted water to a boil. Blanch the green beans for about 2 minutes, then transfer to an ice water bath. Drain and pat dry.
Soak the sliced red onion in ice water for 5 minutes to soften its sharpness, then drain.
In a small bowl, whisk together olive oil, vinegar, garlic, Dijon mustard, honey, salt, and pepper.
In a large bowl, combine the green beans, kidney beans, garbanzo beans, onion, celery, and parsley. Pour the dressing over and toss well.
Refrigerate for at least 30 minutes (or up to overnight) before serving. Bring close to room temperature before serving.