Clean the mushrooms with a cloth and brush - do not wash wild fungi as the flavour disappears. Roughly chop any large mushrooms and leave the small ones intact.
Heat the oil in a frying pan, add garlic & chilli and sweat gently until softened - do not burn or allow to brown. Raise the heat a little, add the mushrooms and stir well for about a minute. Add the stock, keep stirring and cook for a couple of minutes until the liquid has evaporated slightly. Stir in the parsley, some salt and remove from the heat.
Meanwhile place a large saucepan of slightly salted water on the heat and bring to the boil. Add the tagliatelle and cook until "al dente". Drain the pasta and add to the mushroom mixture, stirring well, drizzle with truffle oil and shavings of truffle and serve immediately, with grated parmesan if desired.