Heat the butter or oil in a pan, add the spring onions and courgettes and stir well. Cover and cook for 3 mins, add the peas and stock and return to the boil. Cover and simmer for a further 4 mins, then remove from the heat and stir in the watercress and mint until they are wilted.
Purée in a food processor, adding yogurt with the second batch. Return to the pan, then add seasoning to taste. Serve hot or cold, drizzled with extra yogurt.