In a small, non-reactive (i.e., stainless steel or ceramic bowl), stir together the sliced onion, vinegar, 1/2 teaspoon salt and a few grinds of black pepper. Set aside, agitating occasionally to dissolve the salt, as you prepare the rest of the salad.
Place the prepared potatoes in a large saucepan and cover with 1 inch of cool tap water. Add 3/4 teaspoon salt, and place over high heat. Bring just to a simmer, and lower the heat to maintain a simmer (boiling the potatoes will jostle them around and knock their skins loose). Simmer the potatoes until tender, but not falling apart, about 10 minutes, then drain and let them cool.
Meanwhile, bring a second large saucepan of water to a boil and add 1/2 teaspoon salt. Fill a large bowl with ice and cool water. Cut the tips off of the green beans, and cut the beans on the diagonal into 2" lengths. Drop the prepared green beans into the boiling water, and blanch until bright green and crisp-tender, 30-60 seconds. Scoop the beans out with a slotted spoon, reserving the hot water, and place the beans in the ice bath until cold. Drain well.
Return the green bean water to a boil, and add the shelling beans. Simmer the beans until tender, but not falling apart, about 30 minutes. Drain and cool.
In a very large bowl, combine the potatoes, green beans, shell beans, the red onion-vinegar mixture, capers, and parsley. Drizzle with the olive oil, and toss gently to coat. Taste for seasoning, adding more salt, pepper, or vinegar as needed.
Eat warm, chilled, or at room temperature.The salad is best the day it is made, but will keep for up to a few days in the fridge.
Notes
You can use any fresh bean here; cranberry beans and garbonzos are the ones I see most often. Look for them at a farmer's market or well-stocked hippy co-op. Lacking shelling beans, use 1 cup of dried beans of your choice, (which will take longer to cook).Nutritional values are based on one of six servings.