Bring a large pot of water to a boil then add a palmful of salt - do not skip this step as the salty water will help season the dish. Add the spaghetti then cook until al dente. BEFORE DRAINING remove 1-1/2 - 2 cups of the starchy pasta cooking water then set it aside.

Turn the heat down to medium-low then drain the pasta and return it to the pot. Add the pesto plus a generous splash of the pasta cooking water. Stir with tongs until a creamy sauce has formed, adding more pasta cooking water as needed to create a light and creamy sauce that clings to the pasta. When you stir the pasta you should be able to see a light sauce form in the edges of the pot.

Turn the heat off then add a small amount of fresh lemon juice - the dish should taste brightened, not lemony - plus salt, if needed. Scoop the pasta into bowls then serve with freshly grated parmesan cheese on top.