1. Heat the oil in a large pan over medium-high heat. Add in the onions, celery, garlic, carrots and sweet potatoes and cook for about 15 minutes, stirring often.
3. Add the vegetable stock, quinoa, tomatoes with juice and bay leaves and increase heat to bring to a boil. Once boiling, reduce heat to a simmer and cover, cooking until the vegetables are tender and the quinoa has expanded, about 30 minutes.
4. Pour half of the soup into a food processor and puree until smooth. Return to the pot and mix to combine. Season with salt and pepper.
5. To make the celery pesto: Using a blender or food processor, combine the celery ribs and leaves, parsley, garlic, pistachios, olive oil and Parmesan. Process until smooth and season with salt and pepper.
6. Ladle soup into bowls and top with a spoonful of celery pesto.