Boil a large pot of salted water. Cook pasta until al dente; reserve 1/2 cup pasta water, then drain.
Heat olive oil in a wide skillet over medium-high. Brown the ground beef, breaking it up. Drain excess fat.
Stir in onion and cook until translucent, 3–4 minutes. Add garlic and cook 30 seconds until fragrant.
Pour in marinara and beef broth. Season with Italian seasoning, paprika, red pepper flakes, black pepper, and a pinch of salt. Simmer 5 minutes.
Reduce heat to medium-low. Stir in heavy cream and butter. Simmer gently until slightly thickened, 3–4 minutes.
Add Parmesan and stir until melted and creamy. Adjust salt. Loosen with a splash of pasta water if needed.
Toss in drained pasta and cook 1–2 minutes to coat. Off heat, fold in parsley and mozzarella if using. Serve warm.