Add the pesto into a large serving or mixing bowl and set aside.
Cook the pasta:
Bring a large pot of water to a boil. Once boiling, add the spaghetti and boil according to the package directions. Once the pasta is ready, reserve 1 cup of pasta water.
Drain the pasta water and transfer the pasta into the large bowl containing the pesto. The pot will be too hot and will dry the pasta.
Combine:
Pour half of the reserved pasta water into the bowl with the pesto and pasta. Using kitchen tongs, combine the sauce and slowly add more pasta water until the pesto coats all of the spaghetti noodles and you like the consistency of the sauce. I typically use between ¾ and 1 cup of water total.
Serve:
Serve the pesto spaghetti in pasta bowls topped with Parmesan cheese.