Place the butter and olive oil into a large pot which will fit all of the ingredients later.
Add the onions and garlic, together with a large pinch of salt.
Sauté the onions and garlic over medium-high heat until they have softened slightly.
Add the pumpkin and potatoes.
Pour over just enough stock so that it only just covers the vegetables.
Simmer the vegetables over medium-high heat for about 30 minutes, or until the vegetables are cooked and tender.
Take the pot off the heat.
Blend the soup until you have a smooth mixture. I like to use a stick blender when making soups, but you can also use a jug blender, taking care when handling the hot liquid.
Return the pot to the stove (and return the soup to the pot if you are using a jug blender).
Continue simmering on medium heat.
If the soup looks too thick, thin it with some boiling water.
Taste the soup for seasoning.
Add some freshly cracked pepper and/or nutmeg to taste.
Ladle the soup into some bowls for serving.
Provide some double cream or crème fraîche for everyone to help themselves.