In a large pot of salted boiling water, cook the rigatoni until al dente according to package directions; reserve a cup of pasta water before draining.

Pat the steak chunks dry with paper towels and toss them in Cajun seasoning.

Heat olive oil in a large skillet over medium-high heat, then sear the steak until browned, about 4-5 minutes. Remove from pan and set aside.

In the same skillet, lower the heat and melt butter, then add minced garlic and sauté until fragrant.

Stir in the flour and let it cook for about a minute, then gradually whisk in milk, stirring continuously until slightly thickened.

Slowly stir in the grated Parmesan until melted, adding reserved pasta water as needed to keep the sauce smooth.

Mix in the cooked rigatoni and the seared steak until everything is well-coated.

Garnish with additional Parmesan and chopped parsley before serving immediately.