Bring a large pot of salted water to a boil and cook spaghetti until al dente, then drain and set aside.
Heat olive oil over medium-high heat and brown the ground beef for 6 to 8 minutes, breaking it apart as it cooks.
Drain excess grease, then add the onion and both bell peppers, cooking for 4 minutes until softened.
Stir in 1 tablespoon Cajun seasoning, smoked paprika, salt, and black pepper.
Push the beef mixture to one side and melt 2 tablespoons butter on the other.
Sauté garlic for 1 minute until fragrant, then mix everything together.
Pour in beef broth and heavy cream, stirring gently.
Add cream cheese and whisk until fully melted and silky.
Stir in the remaining Cajun seasoning, Parmesan, mozzarella, cheddar, and red pepper flakes until thick and creamy.
Add the remaining butter and stir until glossy.
Add the cooked spaghetti and toss until every strand is heavily coated and glossy.
Simmer for 2 minutes so the sauce clings to the pasta.
Sprinkle with fresh parsley and serve warm.