Bring a large pot of salted water to a boil. Cook the rigatoni according to package directions until al dente. Before draining, reserve about 1/2 cup of the starchy pasta water. Drain the pasta and set aside.

Heat the olive oil in a large skillet over medium-high heat until shimmering. Add the Cajun-seasoned steak tips in a single layer and sear without moving for 2 minutes to build a crust. Stir and continue cooking until they reach your preferred doneness. Remove from heat and let the steak rest briefly.

Turn the heat to low and pour in the heavy cream. Sprinkle in the grated Parmesan and stir constantly until the cheese melts and the sauce is smooth. If the sauce is too thick, add a splash of the reserved pasta water to loosen it.

Return the drained rigatoni to the skillet, tossing gently to coat the pasta with sauce and steak juices. Heat together for about 1 minute to meld flavors.

Taste and adjust seasoning with salt, black pepper, and additional Cajun seasoning if you want more heat. Serve hot, garnished with chopped parsley.