Sliced Tomato and Onion Salad

1/4 cup balsamic vinegar 
3 tbsp water 
1 tsp olive oil 
1 tsp dijon mustard 
1/4 tsp coarsely ground pepper 
5 medium tomatoes (about 2 1/2 pounds) 
1 large purple onion, thinly sliced and separated into rings 
2 tsp minced fresh oregano 

Combine first 6 ingredients in a jar; cover tightly, and shake vigorously. Set aside. 

Core tomatoes; cut into 1/2 -inch-thick slices. Layer tomato and onion slices in a shallow dish, and sprinkle with oregano. Pour vinegar mixture over tomato and onion slices; cover and marinate in refrigerator 3 hours. 
Yield: 12 servings 

Per serving (73g): 
24 Calories, 0.5g fat, 0g sat fat, 4.3g carbohydrate, 1g fiber,