When the water is boiling, add a generous amount of salt, along with the pasta, and cook until very al dente. Drop the asparagus into the pasta water, gently stir and count to ten. Reserve a cup or two of the pasta water, then drain. Transfer the pasta and asparagus to a serving bowl and toss with the pesto, thinning out with a bit of pasta water as needed to make it saucy. Taste, adjust the seasoning as needed and give an extra squirt of lemon juice to brighten it up, if desired. Garnish with more grated parmesan and chopped walnuts and serve immediately.