Bring a large pot of salted water to a rolling boil.
While the water heats, warm the olive oil, smashed garlic, and crushed red pepper flakes in a small saucepan over medium heat. When the garlic sizzles reduce to medium-low and cook until lightly golden, about 5 minutes; remove from heat.
Transfer the garlic and oil to a blender and set aside.
Add the kale to the boiling water and cook until just tender, about 4–5 minutes. Using tongs, transfer the kale directly to the blender (a little cooking water on the kale is fine).
Add the rigatoni to the boiling water and cook until al dente according to package directions; 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. Season the sauce to taste with salt, pepper, and more lemon juice if desired.
Pour the kale sauce into the pot with the rigatoni, add the Parmesan, and toss to combine. Add reserved pasta water a tablespoon at a time as needed until the sauce is creamy and coats the pasta.
Divide the pasta among bowls and finish with additional Parmesan and freshly ground black pepper to taste.
