Bring a large pot of salted water to a rolling boil.
While the water heats, add the smashed garlic, olive oil, and crushed red pepper flakes to a small saucepan over medium heat.
When the garlic begins to sizzle, lower the heat to medium-low and cook until the garlic is light golden, about 5 minutes; then transfer the garlic and oil to a blender.
Add the kale to the boiling water and cook until just tender, about 5 minutes.
Using tongs, transfer the cooked kale (it can be wet) to the blender with the garlic and oil.
Add the rigatoni to the boiling water and cook until al dente according to package instructions; reserve 1 cup of the pasta cooking water, then drain the pasta and return it to the pot.
Add a splash of the reserved pasta water to the blender along with the lemon zest and 1 teaspoon lemon juice, then blend until smooth.
Taste the sauce and season with salt, pepper, and additional lemon juice as needed; the flavors should be bold.
Pour the kale sauce over the rigatoni in the pot, add the Parmesan, and toss to combine; add more reserved pasta water a little at a time if needed to reach a creamy consistency.
Divide the pasta among 4 bowls and finish with extra Parmesan and freshly ground black pepper to serve.