1. Begin by preparing the fettuccine pasta. Boil a large pot of salted water and cook the pasta according to package instructions until it is al dente. Once cooked, drain the pasta and set it aside.

2. Heat the olive oil in a large skillet over medium-high heat. Meanwhile, season the sliced steak with salt, pepper, and Cajun seasoning. When the oil is shimmering, add the steak strips to the skillet. Sear the meat for about 3-4 minutes, cooking until browned and reaching your preferred doneness. Once cooked, remove the steak from the skillet and keep it warm.

3. Lower the heat to medium and add butter to the skillet. When the butter is melted, add the minced garlic and diced onion. Sauté these ingredients for 2-3 minutes until the onion softens and becomes translucent, and the garlic releases its fragrant aroma.

4. Pour the heavy cream into the skillet, stirring gently, and bring the mixture to a gentle simmer. Allow it to cook for 3-5 minutes until it thickens slightly.

5. Stir in the grated Parmesan cheese, mixing until the sauce is rich, smooth, and creamy.

6. Return the drained pasta and cooked steak strips to the skillet. Toss everything together carefully, ensuring that the pasta and steak are generously coated in the creamy sauce. Heat through for an additional 1-2 minutes.

7. Finally, remove the skillet from the heat and garnish with freshly chopped parsley before serving warm.