1/4 c. vinegar
3 tbsp. water
1 tsp. olive oil
1 tsp. Dijon mustard
1/4 tsp. coarsely ground pepper
1/8 tsp. salt
5 med. unpeeled tomatoes
1 lg. purple onion, thinly sliced & separated into rings
2 tsp. fresh oregano

Combine first 6 ingredients in a jar; cover and shake hard. Set aside. Core tomatoes, cut each into 6 slices, layer tomato and onion in a shallow dish. Sprinkle with oregano. Pour vinegar mixture over vegetables; cover and marinate in refrigerator 3 hours. 12 servings.