Cook the tortellini according to the package directions until just tender. Reserve about ½ cup of the cooking water, then drain carefully.
Melt the butter in a large deep skillet. Add the ground beef and onion and cook over medium-high heat until the beef is deeply browned into small crumbles and the onion is soft.
Stir in the garlic, Cajun seasoning, smoked paprika, garlic powder, black pepper, and salt. Cook for about 30 seconds so the spices coat the beef and become fragrant.
Reduce the heat and add the heavy cream, milk, cream cheese, and Velveeta. Stir gently until the sauce becomes smooth, thick, and pale golden-orange.
Remove from the heat and stir in the mozzarella and Parmesan. Fold in the tortellini carefully until the beef is evenly distributed and every piece is coated. Add cooking water only if needed and finish with parsley.