Bring a large pot of salted water to a boil. Cook pasta according to package instructions until al dente. Reserve 1 cup of pasta water, then drain the pasta.

While pasta cooks, pat the steak dry and season generously with salt, pepper, and 1 tbsp of Cajun seasoning. Heat olive oil and butter in a large skillet over medium-high heat. Sear the steak for 3-4 minutes per side for medium-rare. Remove from skillet and let it rest on a cutting board.

Reduce skillet heat to medium. Add the diced onion and bell pepper and cook for 4-5 minutes until softened. Add the minced garlic and cook for 1 more minute until fragrant.

Pour in the chicken broth to deglaze, scraping up any browned bits. Let it simmer and reduce by half. Stir in the remaining 1 tbsp of Cajun seasoning. Reduce heat to low, then slowly whisk in the heavy cream. Simmer gently for 2-3 minutes to thicken.

Turn off the heat and stir in the grated Parmesan cheese until the sauce is smooth and creamy. Slice the rested steak against the grain. Add the drained pasta and sliced steak to the skillet, tossing to combine everything. If needed, add a splash of reserved pasta water to thin the sauce. Serve immediately, garnished with fresh parsley.