Boil a large pot of salted water and cook spaghetti according to package instructions until al dente; reserve 1/2 cup of pasta water and drain.

Heat olive oil in a large skillet over medium-high heat; add ground beef and diced onion, cooking until beef is browned and onions are translucent.

Drain any excess fat from the skillet, then stir in the minced garlic and Cajun seasoning, cooking for 1 minute until fragrant.

Reduce heat to medium and pour in the heavy cream and beef broth; simmer for 4 to 5 minutes until the liquid begins to thicken.

Turn the heat to low and gradually whisk in the Parmesan, Mozzarella, and Monterey Jack cheeses until the sauce is completely smooth and melted.

Add the cooked spaghetti to the skillet and toss thoroughly to coat the noodles in the sauce, adding reserved pasta water if a thinner consistency is desired.

Season with salt, black pepper, and cayenne pepper to taste; garnish with fresh parsley and serve immediately.