Bring a large pot of salted water to a boil. Add rigatoni pasta and cook according to package instructions until al dente, about 10-12 minutes. Drain and set aside.
In a medium bowl, toss the steak tips with Cajun seasoning until evenly coated.
Heat 1 tablespoon olive oil in a large skillet over medium-high heat. Add the seasoned steak tips and cook, stirring occasionally, until browned and cooked through, about 5-7 minutes. Remove steak tips from skillet and set aside.
In the same skillet, reduce heat to medium and add remaining 1 tablespoon olive oil and butter. Once butter is melted, add minced garlic and sauté for 1 minute until fragrant.
Sprinkle flour over the garlic and butter mixture and whisk continuously for 1-2 minutes to form a roux.
Gradually whisk in the warmed milk, continuing to stir until the sauce thickens, about 4-5 minutes.
Remove skillet from heat and stir in grated Parmesan cheese until melted and smooth.
Season the cheese sauce with salt, black pepper, and crushed red pepper flakes. Adjust seasoning to taste.
Add the cooked rigatoni pasta and Cajun steak tips back into the skillet with the cheese sauce. Toss gently to combine and heat through for 2-3 minutes over low heat.
Sprinkle chopped fresh parsley over the dish before serving.