Peel and de-seed your pumpkin. Cut into 2 inch square pieces.
Place the pumpkin into a bowl and drizzle with the olive oil. Toss to coat well. Arrange in a single layer on a roasting tray and roast for 40 - 60 minutes, until tender.
Heat the butter in a medium sized pan over a medium-high heat. Add the onion and saute gently until translucent, about 5 minutes. Do not allow to brown.
Add the cubed pumpkin, the chicken stock and the sea salt. Simmer, gently, until the vegetables are tender - about 20 minutes.
If not adding the cheese, transfer the soup to a blender and blend until smooth. Add the double cream and season to taste with freshly ground black pepper and blend well. Leave the soup in the blender with the lid on while you fry the sage.
If adding the smoked cheese, return the soup to a clean pan and add the grated cheese before adding the double cream. Gently heat until the cheese has melted and then add the double cream. Set aside and keep warm.
To prepare the fried sage, cover the bottom of a small, non-stick frying pan with sunflower oil. Arrange a few pieces of kitchen towel and the smoked sea salt next to the cooker.
When the oil is very hot, fry the sage leaves for a few seconds on either side.
Using a slotted spoon, transfer the sage leaves from the hot oil to the kitchen towel to drain. Sprinkle liberally with crushed smoked sea salt.
Pour the soup into serving bowls, drizzle with the white truffle infused olive oil and sprinkle with crumbled fried sage. Serve immediately.
Replace the chicken stock with vegetable stock for a vegetarian soup.