Add the sliced mushrooms to the pot and cook until they release their liquid and start to brown, about 10 minutes.
Sprinkle the flour over the mushrooms and stir to combine. Cook for another 2-3 minutes to remove the raw flour taste.
Slowly pour in the vegetable broth while stirring constantly to avoid lumps. Bring the soup to a simmer and cook for 15-20 minutes until slightly thickened.
Stir in the heavy cream and truffle oil, then season with salt and pepper to taste. Simmer for another 5 minutes.
Remove the soup from heat and use an immersion blender to puree the soup until smooth. Alternatively, transfer the soup in batches to a blender and blend until smooth.
Serve the creamy mushroom and truffle soup hot, garnished with chopped fresh chives and a drizzle of truffle oil.