This classic Summer salad features in-season green beans for a fiber, beta-carotene, and vitamin C boost.
In a large bowl, whisk together vinegar and oil; season with salt and pepper. Set aside.
Cook green beans in a pot of boiling salted water until crisp-tender, about 4 minutes. Drain in a colander; pat dry with paper towels.
Add warm green beans, cannellini, and red onion to dressing in bowl; season with salt and pepper. Toss well. Serve, garnished with Parmesan, if desired.