Bring a large pot of salted water to a boil. Add spaghetti and cook until al dente (about 8-9 minutes). Reserve 1/2 cup pasta water, then drain the pasta.

Heat a large skillet over medium-high. Add ground beef and brown, breaking it up. Season with Cajun seasoning, salt, and pepper. Cook until browned, draining excess fat if necessary.

Push the beef to the side, melt butter in the same skillet, then add minced garlic. Sauté for about 30 seconds until fragrant.

Pour in heavy cream and chicken broth. Stir, bring to a gentle simmer, then whisk in softened cream cheese until smooth.

Add cheddar, mozzarella, and grated Parmesan. Stir constantly until velvety and coats the back of a spoon. Add red pepper flakes if desired.

Add cooked spaghetti to the skillet and toss well to coat. Fold in the Cajun beef and toss. If the sauce feels too thick, add reserved pasta water gradually until desired consistency is reached.

Taste and adjust with salt, pepper, or Cajun seasoning. Plate and garnish with parsley, extra Parmesan, and a sprinkle of Cajun seasoning.