Place the chopped red onion in a small bowl of water for about 10 minutes.

Bring a large pot of salted water to a rolling boil.

Add the green beans to the boiling water and blanch for about 2 minutes.

Prepare a large bowl of ice water and transfer the green beans from boiling water into the ice bath.

After a few minutes in the ice bath, drain the green beans and the soaked red onions well.

In a large bowl, toss together the cooled green beans, red onion, and chopped basil. Drizzle olive oil over the top and gently mix.

Pour in balsamic vinegar and sprinkle the freshly grated Parmesan cheese. Toss gently and season with salt and pepper.

Let the salad chill in the refrigerator until ready to serve.