Soak the porcini in the measurement water for about 10 minutes. Drain, reserving the soaking water, and squeeze out any excess water.
Heat the oil in a heavy-based frying pan, add the garlic and thyme and stir for 30 seconds. Increase the heat to high. Add the porcini, season with salt and pepper and stir for 1 minute. Pour in the reserved soaking water and the tomatoes. Bring to the boil, reduce the heat to very low and simmer, uncovered, for 30 minutes until thickened, adding a little water if the sauce begins to stick. Adjust the seasoning.
When the sauce is almost ready, cook the pasta in a large saucepan of salted boiling water until al dente: according to the packet instructions for dried pasta or for 2 minutes if using fresh pasta. Drain thoroughly, reserving a ladleful of the cooking water.
Return the pasta to the pan and place over a low heat. Stir in the mushroom sauce and the butter and mix together thoroughly. Add the reserved pasta cooking water and continue stirring for a few seconds until the pasta is well coated and looks silky. Serve immediately with a scattering of grated Parmesan, if liked.