Clean the mushrooms carefully, brushing to remove any soil. Then rinse quickly under the cold tap to prevent them absorbing too much water. Use a brush to remove traces of soil from the truffle. If using the whole truffle, rinse it quickly under the tap to soften any remaining traces of soil. Then cut the porcini mushrooms into slices 3-4 mm thick. Place the olive oil, butter and crushed garlic in a frying pan and, when the garlic is still pale, add the mushrooms and cook over a medium heat for a maximum of 10 minutes, seasoning to taste. Bring a pan of salted water to the boil and cook the Nonna Rina tagliatelle, drain when al dente and add to the mushrooms, combining well. For a creamier sauce, blend a portion of the mushrooms and mix with the tagliatelle. Use tongs and a ladle to create a nest of tagliatelle and shave the truffle over the pasta using a slicer or grater.