1. Whisk the white wine vinegar, lemon juice, sugar, mustard, lemon zest, red pepper flakes and garlic together in a medium bowl. Continue to whisk and slowly stream in the olive oil. Season with salt and pepper. Stir in the red onion. Set aside.
2. Bring a large pot of water to a boil; prepare a bowl with ice water. Add the green beans and wax beans to the boiling water and cook until tender yet still bright in color and crisp, about 3 minutes. Plunge the beans into the ice water to stop the cooking and drain well. Add the beans to the bowl of dressing and toss to coat. Marinate in the refrigerator at least 2 hours and up to overnight.
3. About 20 minutes before serving, sprinkle the tomatoes with salt and pepper and add them to the green beans. Serve over a bed of chopped lettuce.