Cook green beans in boiling water for about 3 minutes, until slightly tender. Drain and rinse with cold water & pat dry.
Combine beans, onions, tomatoes, and basil in a serving bowl. Whisk oil, sugar, and vinegar in a separate bowl to make dressing. Add salt and pepper to taste.
Toss salad and dressing together. Chill for at least one hour, tossing occasionally. Serve chilled and enjoy!