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Heat the oil in a large saucepan. Add the onion and fry for 8 mins until soft and translucent. Add the garlic and cook for 1 min more. Tip in the broccoli, peas and chard, then pour over the stock and bring the mixture to the boil. Reduce the heat to a simmer, cover and cook for 25 mins.
Stir through the herbs, lemon zest and juice, then blitz the soup with a stick blender until completely smooth. Ladle into bowls and serve with the toasted pumpkin seeds scattered over the top.