Steam the beans for 10 min and put in a bowl in cold water with ice to stop the cooking immediately.
Meanwhile, health a large pan and add the oil when heated. Add the onion and the garlic, and cook over low heat occasionally stirring for 5 minutes.
Drain the beans, and add to the pan, mix thoroughly.
Stir in the tomatoes, season with salt and pepper and discard the garlic if you want to.
Simmer over low heat for about 10 minutes.
Add the olives and basil, and cook for an additional 5 minutes.