Bring a large pot of water to a rapid boil and salt it well. Prepare an ice water bath. Add just enough beans to the water to maintain a boil. Blanch for 1 minute and add to ice bath. Do this in batches until you are done with all the beans. Drain the beans well and put in a serving dish.
Combine onion, mint, champagne vinegar, and olive oil. Season with salt and pepper. Toss to combine and pour over the beans and toss again and serve.