Remove the skin and seeds from the pumpkin, and cut the flesh into large cubes.
Heat the oil in a saucepan; add the potato and cook gently until golden brown add the onion and garlic, mixing them with the diced potato. Add the stock, then the pumpkin and sage, bring to the boil, reduce the heat, cover with a lid and simmer gently for 20-25 minutes, until the potatoes and pumpkin are both soft.
Pour the mixture into a food processor or blender and process to a smooth cream, season with salt and pepper. Return the soup to the saucepan and heat through, swirl in the crème fraiche and spoon the soup into a bowl. Sprinkle the grated cheese on top, and serve.