1) In a large pot of boiling salted water, cook the corn for 3 minutes until the starchiness is gone. Drain and immerse it in ice water to stop cooking and to set the colour.
2) When the corn is cool, cut the kernels off the cob, cutting close to the cob.
3) Toss the kernels in a large bowl with the red onions, vinegar, olive oil, salt, and pepper.
4) Toss in the fresh basil immediately before serving. Taste for seasoning and serve cold or at room temperature.