Blitz the porcini to a powder in a spice grinder or food processor, then stir into a bowl filled with 100ml hot water, to make a paste. Leave to infuse.
Heat the oil and butter in a wide pan over a medium-low heat and fry the onion until golden and translucent.
Add the carrot and celery, fry until softened, stirring regularly, then add the garlic and thyme and fry for another 30 seconds.
Turn up the heat, add the fresh mushrooms and fry, stirring regularly, until they have slumped, browned and no longer give off steam. Add the sherry or wine, sizzle until most of the liquid has evaporated, then add the tomatoes and the porcini paste (taste it first: if it is unpleasantly gritty, dilute it slightly, then strain) and stir well.
Bring to a simmer, then turn down the heat, cover and leave to cook gently for about 10 minutes, until thick.
In the meantime, cook the pasta in plenty of well-salted boiling water, according to the packet instructions.
Season the sauce to taste, then drain the pasta, saving a little of the water. Put the pasta into the pan with the sauce and toss well to coat, adding a splash or two of the reserved cooking water if the sauce seems too thick to coat the pasta evenly. Finish with a drizzle of extra-virgin olive oil and serve with grated parmesan or similar.