Kheera-Tamatar Raita/Yogurt with Cucumber and Tomato

This is adapted from Cooking the Indian Way, page 41.

1 medium tomato
1 medium cucumber, peeled
2 cups plain yogurt (preferably not non-fat)
1 small onion (Vidalia is best), finely chopped
3 Tbsp chopped fresh coriander (cilantro) or parsley
1/2 tsp salt
1/2 tsp black pepper
dash of cayenne pepper (optional)

Cut tomato in half and remove seeds. Chop into small pieces. Chop cucumber into small pieces (optional: remove seeds first by cutting lengthwise and scraping them out with a spoon). Combine all ingredients. Chill at least one hour before serving.