Black Bean and Bok Choy Salad

1 small head bok choy, chopped
2 15-ounce cans black beans, drained
4 celery stalks, diced
2 bunches green onions, white parts only, chopped
3 to 4 tablespoons red wine vinegar
1/2 cup olive oil
2 to 3 tablespoons whole-seed mustard
Salt, to taste
Freshly-ground black pepper, to taste

Combine the bok choy, beans, celery, and onions. Whisk together the vinegar, oil and mustard. Drizzle the dressing over the cabbage and bean mixture and toss. Add salt and pepper to taste. Refrigerate at least 1 hour before serving.