In a large pot, bring salted water to a rolling boil over high heat. Add spaghetti and cook according to package instructions until al dente, usually about 8-10 minutes. Drain the spaghetti and set it aside.

Toss diced sirloin or ribeye steak with Cajun seasoning until well-coated. In a large skillet, heat butter over medium-high heat. Add the seasoned beef, searing it for about 4-5 minutes until browned. Remove the beef from the skillet.

In the same skillet, reduce heat to medium. Add minced garlic and sauté for about 1 minute. Pour in heavy cream and add cream cheese, mozzarella, and Parmesan. Stir continuously for 3-5 minutes until the sauce is smooth.

Return the browned Cajun beef to the skillet along with the drained spaghetti. Toss everything together gently for 2-3 minutes until well-coated with the sauce.

Remove from heat and sprinkle with chopped fresh parsley for garnish. Serve hot, optionally topping with extra Parmesan or Cajun seasoning.