In a medium saucepan melt the butter, then add the sliced leek and spring onions. Stir them around, add some salt, put the lid on and cook gently for 5 minutes. After that, add the frozen potato, stock and milk and bring everything up to simmering point.
Give it one good stir, cover the pan and simmer gently for 20 minutes. Then, off the heat, add the watercress, spinach and rocket leaves, then gently whiz the soup to a purée with a stick blender – though not too much, you still need to see bits of leaves. Taste and check the seasoning before serving, garnished with the