In a soup pot over medium heat, warm oil. Add onions and cook until soft. Add green peppers, celery, tomatoes, and salt and pepper. Reduce heat to medium-low and cook for 30 minutes. Add boiling water and increase heat. Cook for 5 minutes. Add lemon zest and juice. Remove pot from heat and cool slightly. Press through a food mill and serve individual portions topped with yogurt.
Soup can be served warm or cold.