Heat the butter or bacon grease in a large Dutch Oven or heavy stock pot. Slice the onion thinly - I used a mandoline. Add the onions and cook over low heat for about 20 minutes, stirring occasionally. When the onions become deep golden brown, add the broth and cauliflower. Simmer for about 10 minutes or until the cauliflower is fork tender.Puree with an immersion blender (or transfer to a regular blender) until the soup reaches your desired consistency.Add the cheese and stir until melted. Add the truffle oil and stir to incorporate. Sometimes I give it one last whir with the immersion blender to get the cheese and truffle oil distributed evenly.