Flavors meld as this soup sits, and it tastes even better the second day.
Heat oil in soup kettle. Saute onions until soft. Add green peppers, celery, tomatoes, and salt and pepper, and cook over moderate heat 30 minutes. Add boiling water and increase heat. Cook 5 minutes. Add lemon rind and juice. Cool slightly. Press through food mill, reheat, and serve garnished with yogurt and chopped dill, parsley, or chives. (Also good cold.)