In a casserole-type pan warm the olive oil and butter together and stir fry the leek and spring onions for 3 minutes. Chop the potatoes and carrots finely and add them in. Leave for 3 minutes to glaze, then, add the meat stock. Simmer for 25 minutes. Remove from heat and blend the soup until smooth. Return to the heat, add the herbs and simmer for another 5 minutes. Serve with yoghurt.