In a saute pan on medium high heat, melt a stick of butter with the sliced onion. When the onion starts to brown, turn the heat down to low and cook for about thirty to forty minutes stirring a few times to prevent burning. Add splashes of white wine to deglaze as the onions carmelize. Meanwhile, clean and chop the mushrooms and set aside. In a stockpot, gently heat the pumpkin mousse on low with the vegetable stock, stir in the carmelized onions keeping the pan you cooked them in on the side, and puree in a mixer working in batches. Put all the pureed soup back into the stockpot. In the same pan you used to cook the onions, saute the chopped mushrooms for about ten minutes on medium low heat and set them aside. Season with salt and pepper until you're happy with the taste. Serve the soup with a couple of tablespoons of mushrooms and drizzle with the truffle oil.