Cook the spaghetti according to package instructions until al dente. Drain and set aside.
In a large skillet, heat olive oil over medium heat. Add the diced onion and sauté for about 3 minutes until softened.
Add the minced garlic and ground beef to the skillet. Cook until the beef is browned and cooked through, breaking it apart with a spoon as it cooks.
Stir in the Cajun seasoning and diced tomatoes (with their juice). Cook for another 5 minutes, allowing the flavors to meld.
Reduce the heat to low and pour in the heavy cream. Stir well to combine.
Add the mozzarella, Parmesan, and cheddar cheeses to the skillet. Stir until the cheeses are melted and the sauce is creamy. Season with salt and pepper to taste.
Add the cooked spaghetti to the skillet and toss to coat the pasta evenly with the sauce.
Serve hot, garnished with chopped fresh parsley.