Melt butter in stock pot. Add chopped onion and garlic and sauté until soft and the fumes coming off lose their acridity. Add the cumin and cook for a minute or so. Add the pumpkin, sweet potato and stock. Cover and bring to the boil. Simmer covered for 15-20 minutes, until the vegetables are soft. Blend the contents of the pan with a stick blender until the soup is smooth, being careful not to splash the hot liquid and burn yourself. Season with pepper to taste and stir. Serve.
The quantities are loose. The recipe should still be okay with up to half the amount of vegetables. The stock can also be augmented with up to the same volume of water, but the quantities given make a nice thick soup.
Recipe makes 4-5 bowls