Cook the linguine according to the instructions on the packet.
Meanwhile, heat the butter in a frying pan. Add the mushrooms until soft and golden and any liquid has almost all evaporated. Add the garlic, thyme leaves and chilli flakes and cook for a couple more minutes. Season well.
Drain the linguine, reserving a cupful of the cooking water. Add the linguine to the mushrooms along with the lemon juice, parmesan and parsley. Toss everything together and add enough of the reserved cooking water to form a light sauce. Season to taste and serve.