Cook the pasta in salted water according to package directions until al dente. Reserve some pasta water, then drain.

Season the steak with AP and Cajun seasoning. Sear in a hot oven-safe skillet until browned. Add ½ stick butter, rosemary and thyme; baste with melted butter. Finish in the oven to your preferred doneness, then rest before slicing.

Melt 2 tablespoons butter in a pan. Cook the onion until softened, then add garlic and cook until fragrant.

Add heavy cream, AP and Cajun seasoning, and red pepper flakes to taste. Simmer gently, reduce heat and stir in Parmesan until smooth. Add reserved pasta water as needed.

Toss the pasta in the sauce. Slice the steak and serve over pasta with parsley and additional Parmesan.