Cook the Pasta: Bring a pot of salted water to a boil and cook rigatoni until al dente. Reserve 1/4 cup pasta water, then drain.
Cook the Steak: Pat steak dry and season with Cajun seasoning and salt. Heat olive oil in a skillet over high heat and sear steak for 2β3 minutes per side until cooked to preference. Remove, let rest for 5 minutes, then cut into cubes.
Make the Sauce: Lower heat to medium and add butter to the same pan. Add garlic and cook for about 30 seconds until fragrant. Stir in crushed tomatoes, cream, Cajun seasoning, and black pepper. Simmer for 3β4 minutes until slightly thickened.
Combine Pasta and Sauce: Add cooked rigatoni to the sauce with a splash of pasta water. Toss until glossy and well coated.
Assemble the Dish: Transfer pasta to a bowl and top with Cajun steak pieces.
Finish and Serve: Sprinkle with parmesan, parsley, and red pepper flakes if desired. Serve immediately.