In a medium bowl, cover the porcini with boiling water and leave to soak for 10 minutes. Drain, reserving 75g of the soaking liquid. Very finely chop the porcini to mince consistency, then set aside.
Put the oil, garlic, chilli flakes, parsley and fine salt into a cold, large saute pan on a medium-low heat. Very gently fry for 5 minutes until soft and lightly golden, turning the heat down if the garlic starts to brown.
Increase the heat to medium-high, then add the chopped porcini, tomato paste and plenty of pepper. Stir-fry for 3 minutes, then set the pan aside while you boil the pasta.
Cook the pasta in salted boiling water for about 6 minutes, until al dente. Drain, reserving 350g of the pasta water.
Return the saute pan with the porcini to a medium-high heat, then add the 350g of pasta water and the reserved 75g of porcini soaking liquid. Stir and bring to a simmer. Once simmering, leave to bubble away for 3 minutes.
Add half the parmesan to the pan, stirring until it has melted before adding the rest. Lower the heat to medium, then stir in the cream, followed by the drained tagliatelle. Toss over the heat until the pasta and sauce have emulsified – about 1½ minutes.
Remove from the heat and serve at once, finished with as much extra oil and parmesan as you like