Bring a large pot of salted water to a boil and cook the spaghetti until al dente, following the package time.

Finely zest the lemon and chop the fresh parsley.

Whisk the egg yolk, lemon zest, grated Parmesan cheese, heavy cream, salt, and black pepper in a mixing bowl until smooth.

Reserve 60 ml of pasta cooking water about 3 minutes before the spaghetti finishes cooking and whisk it into the sauce mixture.

Transfer the cooked spaghetti directly to a frying pan over low heat and add the lemon-Parmesan sauce.

Toss the spaghetti vigorously until the sauce emulsifies and evenly coats the pasta.

Add a few drops of lemon juice and the chopped parsley, then toss briefly.

Serve immediately while the sauce is creamy.

Serve immediately.