Bring a large pot of salted water to a boil for the pasta. Meanwhile, heat the olive oil in a large skillet over medium heat. Add the grated zucchini and a pinch of salt; cook, stirring occasionally, until softened and beginning to release liquid, about 5 minutes. Stir in the garlic and cook 1 minute more, until fragrant.

Push the zucchini to the side of the pan and add the ricotta. Warm for 30 seconds, then stir to combine. Add the lemon zest and half of the Parmesan. Pour in about 1/2 cup of the hot pasta water and stir until smooth and creamy. Season with salt and pepper.

Cook the pasta until al dente. Reserve another cup of pasta water, then drain. Add the hot pasta to the skillet with the lemon juice and toss, adding splashes of pasta water as needed, until the sauce coats the noodles, about 1 minute.

Divide among bowls and top with the remaining Parmesan, torn basil, and red pepper flakes if using. Serve immediately.