Put the dried porcini mushrooms in a bowl and cover with the boiling water. Set aside to soak for 15 minutes, then drain, reserving the soaking liquid. Slice the rehydrated mushrooms, discarding any tough stalks.
Heat half the olive oil in a saucepan, then add the shallots and sauté for 3 minutes or until they are tender and golden. Add the button mushrooms and cook for 8–10 minutes or until all the juice from the mushrooms has evaporated.
Add the Marsala and the soaking liquid from the dried mushrooms. Simmer for about 10 minutes or until the liquid has reduced by half.
Meanwhile, cook the tagliatelle in a saucepan of boiling water according to the packet instructions.
About 5 minutes before the pasta is ready, heat the remaining oil in a large frying pan. Add the garlic, if using, and the mixed mushrooms. Cook over a medium heat, shaking the pan often, for 3–5 minutes or until the mushrooms are lightly cooked.
Stir the tomatoes, thyme and parsley into the mushroom sauce and heat through for 1–2 minutes. Add the mixed mushrooms, season with pepper and remove from the heat.
Drain the tagliatelle and divide it among four serving bowls. Spoon the mushroom sauce on top and serve.