1.Using a mandoline, thinly slice the fennel and transfer it to a large bowl. Thinly slice the cremini mushrooms and set aside.
2.In a small bowl, whisk the olive oil with the lemon juice. Season generously with salt and pepper. Pour all but 1 tablespoon of the dressing over the fennel; season with salt and pepper and toss to coat.
3.Mound the sliced fennel on plates and top with the sliced cremini. Shave Parmesan over the salad. Drizzle with the remaining 1 tablespoon of dressing and the truffle oil and serve.