In a large saucepan, heat the butter and olive oil.
When it starts foaming, add the spring onions. Fry until golden and then stir in the stock and bring to the boil. Stir in the peas and milk, simmer for about 5 minutes. Remove the pan from the heat and add the mint leaves. Blend in a blender until smooth. Serve hot or cold with grated parmesan.