Boil a pot of salted water. Add the green beans and cook until tender-crisp, 1 to 2 minutes. Drain the water and move beans to a bowl of ice water to cool. Drain well, pat dry and place the beans in a bowl. Combine with the tomatoes and red onions.
Whisk together the olive oil, red wine vinegar, basil, garlic, and a pinch of salt and pepper. Pour the dressing over the green beans. Let it marinate in the refrigerator until you are ready to serve.