Start by bringing a large pot of salted water to a boil. Add the rigatoni and cook it until al dente according to the package instructions. Before draining,
While the pasta cooks, heat olive oil in a large skillet over medium heat. Add the sliced zucchini and cook until tender and lightly golden. Stir in the minced garlic and red pepper flakes, cooking just until fragrant. Be careful not to burn the garlic.
Add the drained pasta directly into the skillet with the zucchini mixture. Sprinkle in the lemon zest, lemon juice, and half of the grated Parmesan. Toss everything together while slowly adding reserved pasta water. Continue mixing until the sauce becomes silky and coats the pasta evenly.
Season with salt and black pepper to taste. Remove from heat and sprinkle the remaining Parmesan and fresh parsley on top. Serve immediately while warm for the best flavor and texture.