Bring a large pot of water to a boil.

In another bowl add cold water and ice to make an ice bath.

Use a small knife to score a small "x" in the bottom of the onion. Once water begins boiling, boil onions 3 - 5 minutes or until slightly tender. Use a slotted spoon to transfer the onions to the ice bath. This will stop the cooking process. Once cooled simply squeeze and the onion should pop out of the skin. Cut the onions in half.

In a large skillet heat olive oil over medium heat. Saute green beans, salt and pepper for 2 minutes. Pour in vinegar. Cover beans and continue to cook for 10 minutes. Stir in onions and tomatoes. Continue cooking just until the vegetables are tender about 5 more minutes. Adjust seasoning with more salt and pepper if desired. Serve.