1. Season the steak generously with Cajun seasoning and let it rest at room temperature for about 10 minutes.
2. Heat a skillet over medium-high heat with olive oil and butter, then sear the steak for 3-5 minutes per side until cooked to desired doneness.
3. Remove the steak from the skillet, let it rest for 5-10 minutes, then slice it thinly.
4. Cook the fettuccine pasta according to package instructions until al dente, then drain and set aside.
5. In the same skillet, melt butter over medium heat and sauté chopped onions and minced garlic until fragrant.
6. Pour in the heavy cream and bring to a gentle simmer while stirring continuously.
7. Gradually add grated Parmesan cheese, stirring until fully melted and smooth.
8. Sprinkle in Cajun seasoning to taste, adjusting the spice level as needed.
9. Add the cooked fettuccine to the sauce, tossing to coat evenly.
10. Arrange the sliced steak on top of the pasta and gently mix if desired.
11. Garnish with chopped fresh parsley for extra flavor and color.
12. Serve immediately with garlic bread and a light salad.