In a large saucepan, heat 1 tbsp of olive oil. Add the onion and celery and fry for 10-15 minutes, or until soft. Add garlic and cook for another minute. Stir in all the other ingredients, apart from pesto and remaining oil, and bring to boil.
Reduce the heat and leave to simmer for 6-8 minutes, or until orzo is just tender. Season to taste and ladle to bowls. Stir in the remaining oil with pesto and drizzle over the soup. Serve with crusty bread on side.