lace the dried porcini in a jug and pour in the boiling water. Allow to stand for 5 minutes. Meanwhile, place a large frying pan on a medium heat and add the oil, onion and garlic.
Add a pinch of salt and sauté for 5–10 minutes until softened but not coloured. Strain the porcini, reserving the liquid.
Add them to the pan with the thyme and fresh mushrooms, then turn up the heat. Stir-fry for 15 minutes until the mushrooms are soft and golden.
Discard the thyme sprigs. Add the wine and allow to bubble and reduce by half. Cook the tagliatelle in a large pan of boiling, salted water according to the packet instructions.
Towards the end of the pasta cooking time, add the butter to the mushroom pan, stir well and allow to bubble. Stir in the crème fraîche, turn the heat down and thicken a little.
Season to taste and add a little porcini water. Drain the pasta, reserving a cupful of the cooking water, and tip into the mushroom pan.
Toss the pasta in the sauce and loosen with pasta water if required. Divide between bowls, drizzle over a few drops of truffle oil, then sprinkle over some Parmesan and extra thyme leaves and serve.