Start by boiling a pot of salted water. Add the rigatoni pasta and cook according to package instructions until al dente. Reserve ½ cup pasta water, then drain and set aside.

In a large skillet over medium-high heat, melt the butter. Add the steak tips, season with Cajun seasoning, salt, and pepper. Sear until browned, about 5-7 minutes. Remove from the skillet and set aside.

In the same skillet, add the diced onion and minced garlic. Sauté until fragrant and softened, about 2-3 minutes. Stir in the heavy cream and bring it to a gentle simmer. Gradually add the grated Parmesan cheese, stirring until melted and blended.

Return the seared steak tips to the skillet and toss them in the creamy sauce. Add the cooked rigatoni and mix thoroughly to coat. If the sauce is too thick, gradually incorporate the reserved pasta water until desired consistency is reached.

Taste and adjust seasoning if needed, then serve hot, garnished with additional Parmesan cheese if desired. Enjoy with your favorite bread or side salad!