Mix the sauce. In a large bowl, whisk the ricotta, Parmesan, lemon zest, olive oil, and half the lemon juice until smooth.

Cook the rigatoni. Cook in salted boiling water until al dente. Reserve a mug of the cooking water before draining.

Loosen and toss. Whisk a little hot pasta water into the ricotta mixture. Add the pasta and toss well, adding more water until the sauce coats every piece.

Taste and finish. Fold in the parsley. Taste for salt, pepper, and the remaining lemon juice. Serve immediately.