Long a staple of the picnic table and salad bar, this classic is simple to make, tasty, and reliably inexpensive. It’s often soggy and loaded with sugar when you buy it, but when you make it yourself, it has a fresh, beany crunch.
In a large mixing bowl, whisk together the vinegar, olive oil, mustard, salt, and pepper. Cook the fresh beans in a small amount of salted water, just until crisp-tender, about 5 minutes. Drain and rinse the kidney beans. Toss the hot beans, the drained beans, and the onion, pepper, and parsley with the dressing. Add salt, if necessary. Chill, covered, to let the flavors combine.