Melt the butter in a large pot, add the onion and butternut and cook for 5 minutes.
Sprinkle the flour over the vegetables and cook for a further 3 minutes, stirring all the time.
Add the heated stock and Wild Appetite Cooks Essential Sauce and allow the soup to simmer until tender.
Cool slightly and puree in a blender.
Season with salt and pepper and garnish with plain yoghurt when serving.