Bring a large pot of salted water to a rolling boil. Add spaghetti and cook according to package instructions until al dente, about 8-10 minutes. Drain the pasta, reserving a small cup of pasta water.

In a large skillet, heat 2 tablespoons of butter over medium-high heat until melted. Toss diced sirloin or ribeye with Cajun seasoning, then add the beef to the skillet and sear for about 4-5 minutes.

Lower the heat and add more butter if needed. Once melted, add minced garlic and sauté for about 1 minute until fragrant.

Pour in 1 cup of heavy cream, then add cream cheese, shredded mozzarella, and Parmesan. Stir continuously until the mixture simmers and thickens, about 3-4 minutes.

Lower the heat to medium-low and add the seared beef and cooked spaghetti to the skillet. Toss until well-coated and heated through, about 2-3 minutes.

Serve hot, garnished with chopped parsley and extra Parmesan if desired.