Heat the oil in a large saucepan and sauté the onion, garlic and celery over a gentle heat for 5 minutes until soft but brown.
Add the finely diced potato and tomato and stir through, then add the stock.
Bring the mixture to the boil then simmer for 10 minutes until the potato is soft.
Add the vegetables and haricot beans and cook for a further 3 minutes until the vegetables are soft.
Season with black pepper and serve the soup straight away with a spoonful of pesto in each bowl and some crusty bread