For presentation purposes, remove 2 2-inches strips of lemon zest Using a vegetable peeler. Cut into thin strands. Set aside.

Finely grate remaining zest into a pot.

Cut lemon in half and squeeze out enough juice to yield 2 Tbsp. into a small bowl. Set aside.

Add cream to pot with lemon zest and cook over medium heat, whisking often until liquid is just beginning to simmer. Reduce heat to medium-low then whisk in butter 1 Tbsp. at a time until melted and sauce is creamy and emulsified. Remove from heat.

Add ¾ cup pasta cooking liquid to cream sauce and return to medium heat. Stir to combine then transfer spaghetti to pot with sauce. Cook, tossing often and adding Parmesan little by little, until cheese is melted and sauce is creamy, about 3 minutes. If sauce looks tight, add 1–2 Tbsp. pasta cooking liquid.

Stir in reserved lemon juice; season with more salt, if needed.

Serve with the lemon zest strands.