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Heat the oil in a flameproof casserole or saucepan over a medium heat and fry the onion and celery with a pinch of salt for 10 mins until beginning to soften. Add the potatoes, broccoli and stock, and bring to a simmer. Cover and cook for 20-25 mins, or until the thick broccoli stalks are tender enough to pierce with a cutlery knife. Remove from the heat and add half the buttermilk and 80g of the blue cheese. Blitz everything together using a hand blender until smooth. Season to taste, then stir in the cream, remaining buttermilk and the lemon juice.
Ladle the soup into the bowls and top with the rest of the cheese, the seeds, chives and a drizzle of olive oil, then serve.
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