1) Bring a large pot of salted water to a boil. Add the farfalle and cook until al dente, stirring occasionally, about 12 mins. Drain, reserving 240ml of pasta water.
2) 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 mins. Add the asparagus and saute until the asparagus is crisp-tender, about 5 mins. Add the farfalle. Stir in the mascarpone and nutmeg and toss until the cheese coats the pasta, adding the reserved cooking liquid 60ml at a time to moisten. Stir in 50g walnuts. Season the pasta, to taste, with salt and pepper. Mound the pasta in a large bowl. Sprinkle with the Parmesan and remaining 25g walnuts. Serve.