Bring a large pot of salted water to a boil and cook spaghetti according to package directions until al dente. Reserve 0.5 cup of pasta water before draining.
Heat olive oil in a large skillet over medium-high heat. Add ground beef and brown until fully cooked, breaking it into small crumbles. Drain excess fat.
Add the diced onion to the skillet and sauté for 4 minutes until translucent.
Stir in minced garlic, Cajun seasoning, and smoked paprika. Cook for 60 seconds until fragrant.
Reduce heat to medium and pour in the heavy cream. Bring the mixture to a gentle simmer.
Gradually whisk in the Parmesan, Mozzarella, and Gruyère cheeses. Stir constantly until the cheeses are fully melted and the sauce is smooth.
Incorporate the cooked spaghetti into the sauce, adding the reserved pasta water 1 tablespoon at a time if needed to adjust the consistency.
Season with salt and pepper to taste. Garnish with chopped parsley and serve immediately.