1. Stir together tomato, shallots, vinegar and mustard in a large bowl. Let stand 10 minutes.
2. Heat a pan to med-high heat. Add onion and cook for two minutes until soft. Remove from pan and let cool.
3. Steam green beans for 3 to 4 minutes or until crisp and tender. Plunge beans into ice water to stop the cooking process; drain and pat dry.
4. Toss together beans, sautéed onions and tomato mixture. Serve at room temperature, or cover and chill up to 2 hours.

Notes: This salad can also be prepared with Kalamata olives.