Tuna, cannellini beans, green beans, and red onion are matches made in salad heaven -- toss them in a simple dressing of white-wine vinegar and olive oil for a light and refreshing lunch or dinner.
In a medium pot of boiling salted water, cook green beans until bright green and crisp-tender, about 3 minutes. Drain, rinse under cool water to stop the cooking, and pat dry.
In a large bowl, whisk together vinegar and oil and season to taste with salt and pepper. Add green beans, cannellini beans, onion, tuna, and pepperoncini and stir to combine. Serve immediately.