Heat the oil in a large saucepan, add the onion, celery and a pinch of salt, and fry for 8 mins until soft. Add the stock with the zest and juice of the lemons, and season. Stir in the orzo and cook for 5 mins, then add the peas and spinach, and cook for a further 5 mins. Swirl though the pesto and season.
Heat the flatbreads, if using, following pack instructions. Ladle the soup generously into bowls and top with a handful of parmesan. Serve with the flatbread to dip.