In a large pot, bring a generous amount of salted water to a rolling boil. Carefully add the rigatoni and cook according to the package instructions, usually about 10-12 minutes, until it reaches al dente texture. Once cooked, drain the pasta in a colander, but remember to reserve 1/2 cup of the pasta cooking water for later.
In a medium mixing bowl, add the sliced flank steak. Sprinkle the Cajun seasoning over the beef and toss well to ensure each piece is evenly coated with the spicy seasoning blend.
Heat the olive oil in a large skillet over medium-high heat. Once the oil is shimmering, carefully add the seasoned steak strips. Sear the steak for about 3-4 minutes, or until nicely browned and cooked to your desired doneness. Remove the steak from the skillet and transfer to a plate to rest.
In the same skillet used for the steak, lower the heat to medium and add the minced garlic. Sauté for 30 seconds, stirring frequently, until the garlic is fragrant and slightly golden. Next, slowly pour in the heavy cream while stirring, bringing the mixture to a gentle simmer.
Gradually mix in the grated Parmesan cheese, stirring continuously until it melts and the sauce thickens to a creamy consistency. If the sauce becomes too thick, add the reserved pasta water a little at a time, stirring until you achieve your preferred consistency.
Add the cooked rigatoni and the fresh spinach to the Alfredo sauce. Stir the mixture gently until the spinach wilts and the pasta is well coated with the creamy sauce. Finally, fold in the seared steak and season the dish with salt and fresh black pepper to taste.
To dish up, portion the Cajun Steak Rigatoni Alfredo onto serving plates. Garnish each plate with a sprinkle of finely chopped parsley for a splash of color and freshness. Enjoy your delightful meal!