Cover porcini mushrooms with boiling water and set aside for 20-30 mins. Then drain, reserving the liquid, but leaving any grit behind.
Toast the walnuts in a dry pan. Remove and set aside.
At the same time, cook penne in plenty of lightly salted boiling water until al dente. Drain well.
Heat a little oil in a heavy-bottomed pan and gently saute red onion and garlic until softish.
Add the button mushrooms. Stir well and then add the porcini soaking liquid, together with the porcinis, a little stock if needed, seasonings and thyme. Mix and briefly cook. Taste for seasoning.
Then toss together the drained pasta, walnuts and the sauce.
To serve, mound the pasta in individual flat soup bowls and sprinkle with parmesan.
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