Prepare asparagus by washing and trimming off the woody ends. Chop diagonal into about 1/3-inch (1 cm) thick slices.
In a medium pot on high. Boil water and cook the pasta according to the packaging instructions.
In a large pan, on medium heat, add olive oil and sauté the onion until soft, about 5 minutes.
Add asparagus to pan and sauté for 5 minutes. Add a bit of lemon zest and stir.
Set aside some pasta water. Drain pasta and let pasta sit in the colander while making the sauce.
Add cream cheese, some lemon zest (keep some for topping the finished dish), and black pepper to the empty pasta pot. Heat until cream cheese becomes liquid, while stirring. Add 1 tablespoon pasta cooking water at a time, to get a creamy sauce.
Squeeze some lemon juice over the onions and asparagus in the pan, then stir together. Add the drained pasta and vegetables to the pot with the sauce and stir to combine.
Serve the pasta with some lemon zest, parmesan, and some red pepper flakes and black pepper if desired on top.