Bring a large pot of heavily salted water to a boil. While it heats, grate the zucchini on the large holes of a box grater directly into the center of a clean kitchen towel. Gather the towel up around the zucchini and squeeze firmly over the sink until no more liquid comes out. This step matters. Skipping it will make your sauce watery. Set the squeezed zucchini aside.

In a large mixing bowl, combine the ricotta, lemon zest, lemon juice, grated garlic, Parmesan, red pepper flakes, 1 teaspoon salt, and black pepper. Stir together until smooth and well combined.

Add the squeezed shredded zucchini to the ricotta mixture and stir it in evenly. Taste and adjust salt if needed. Set the bowl aside at room temperature while the pasta cooks.

Cook the rigatoni in the boiling salted water according to package directions until just al dente. Right before draining, scoop out about 1/2 cup of the hot starchy pasta water and set it aside. Drain the pasta.

Immediately add the hot drained rigatoni to the bowl with the ricotta mixture. Toss quickly with tongs to coat. The heat from the pasta will warm the sauce and make it turn creamy and loose. Add pasta water a splash at a time, tossing between each addition, until the sauce coats the pasta evenly and flows slightly. You may not need all of the reserved water.

Serve right away in warm bowls with extra Parmesan and a few more red pepper flakes on top if you like. This dish is best eaten fresh while the sauce is still creamy and the pasta is hot.