Pat the sirloin steak dry with paper towels and season both sides liberally with Cajun seasoning, garlic powder, salt, and pepper. Allow to rest for 10-15 minutes.

In a large pot, bring salted water to a rolling boil. Add the rotini pasta and cook according to package instructions until al dente, about 8-10 minutes. Reserve ¼ cup of pasta water, then drain the pasta.

Heat olive oil in a cast iron skillet over medium-high heat until shimmering. Add the steak and sear for 3-4 minutes on each side. In the last minute, add butter to baste the steak. Remove from heat and let rest for 5 minutes.

In a medium saucepan, melt 2 tablespoons of butter over medium heat. Add minced garlic and sauté for 1-2 minutes until fragrant. Stir in flour and cook for another minute to thicken.

Slowly whisk in heavy cream. Reduce heat to low and stir in parmesan cheese, black pepper, Italian seasoning, and salt until melted and creamy, about 3-4 minutes.

Add the drained pasta to the garlic parmesan sauce and toss gently. Adjust consistency with reserved pasta water as needed.

Plate the pasta and top with sliced Cajun steak. Drizzle with pan juices and garnish with chopped parsley. Serve hot.