Bring a large pot of well-salted water to a rolling boil. Add spaghetti and cook until al dente (usually 8–10 minutes). Reserve 1/2 cup pasta water, then drain and set aside.

While the pasta cooks, toss the diced steak with Cajun seasoning until evenly coated.

Heat a heavy skillet over high heat. Add butter and let it foam. In batches if necessary, add the beef in a single layer. Sear without moving for 1–2 minutes, then stir and cook until browned and cooked to your liking (another 1–2 minutes). Remove beef to a plate and keep warm.

Reduce heat to medium. Add minced garlic to the same skillet and sauté for about 30–60 seconds until fragrant—don’t let it burn.

Pour in the heavy cream and bring to a gentle simmer. Stir in cream cheese in chunks so it melts evenly. Add mozzarella and grated Parmesan, stirring constantly until the sauce is smooth. If the sauce looks too thick, whisk in a splash of reserved pasta water to loosen it.

Return the seared beef and the cooked spaghetti to the skillet. Toss everything gently but thoroughly so the noodles are coated in the cheese sauce. Heat through for 1–2 minutes to marry flavors. Taste and adjust seasoning with salt, pepper, or more Cajun seasoning.

Remove from heat. Sprinkle with chopped parsley and extra Parmesan. Serve immediately with additional Cajun spice at the table if guests want more heat.