Heat one tablespoon of olive oil in a large deep pan or in a large saucepan, then add finely diced celery and shallot, saute over low heat for 10 minutes. Meanwhile, add the remaining tablespoon of olive oil in a frying pan and fry the mushrooms whole or sliced in half if very large over medium high heat for 5 minutes or until golden on the outside, season with salt and cook for a couple of minutes longer. Take off the heat and set aside till needed.
Once the celery and shallot are softened, add the rice and toast for a couple of minutes until every grain is evenly coated with oil. Pour in the white wine and simmer for a minute until nearly evaporated.
Pour in a quarter of the stock, so it is just above the rice level and stir, then add the cooked mushrooms and let everything simmer for a few minutes over medium heat. The water should be just above the rice when you first add it. Once you see the water going down add a bit more and stir. Repeat until all water is gone and the rice is cooked but still slightly chewy. Give it a quick stir every time you add more water or stock if using.
When the rice is cooked to your liking, stir in the black truffle slices, Parmesan cheese, a tablespoon of Mascarpone cheese or butter, stir one more time, cover with a lid and take it off the heat. Let it sit for 5 minutes, then uncover and serve.