In a large pot, melt the butter and add the onions. Cook until the onions are translucent, about 8 minutes. Add the stock and the squash and simmer until the squash is tender. With a blender, food processor or immersion blended, blend the squash and onions until smooth. Return to pot. Season with salt, pepper and nutmeg.
With this basic recipe, I like to garnish each bowl with a dollop of sour cream. Croutons could be added to the bowls. Other variations I have made are to add a peeled and cored apple to the squash or add a couple of teaspoons of madras curry. Crumbled fried bacon also makes a nice topping.