Soak the porcini in 200 ml hot water for 20 minutes. Finely grate the peel from half the lemon and squeeze all the juice.
Drain the porcini, reserving the water. Rinse the porcini and chop. Strain the liquid through muslin to remove any grit.
Melt the butter in a thick bottomed pan and add the garlic, sage and chilli. Colour, then add the porcini. Fry until soft then add 4 tbsps of the liquid and simmer until most of the juice has been absorbed.
Add the cream, lemon zest and juice, and reduce until the sauce is creamy and thick. Season.
Cook the tagliatelle in boiling salted water until al dente then drain. Add to the sauce and turn over to coat thoroughly. Serve with the parmesan.