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.