Bring a large pot of salted water to a boil. Add the farfalle and cook until al dente, stirring occasionally, about 12 minutes. Drain, reserving 1 cup of pasta water.
Meanwhile, melt the butter in a heavy large skillet over medium heat. Add the mushrooms and saute until tender and most of the juices have evaporated, about 5 minutes. Add the asparagus and saute until the asparagus is crisp-tender, about 5 minutes. Add the farfalle. Stir in the mascarpone and nutmeg and toss until the cheese coats the pasta, adding the reserved cooking liquid 1/4 cup at a time to moisten. Stir in 1/2 cup of walnuts. Season the pasta, to taste, with salt and pepper. Mound the pasta in a large bowl. Sprinkle with the Parmesan and remaining 1/4 cup of walnuts. Serve.
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