Cook the fettuccine in salted boiling water until al dente. Save 1/2 cup pasta water, then drain.
Heat olive oil in a large skillet over medium-high heat. Add onion and ground beef, breaking the meat into chunky crumbles. Cook until nicely browned, then drain excess grease.
Add 2 minced garlic cloves, tomato paste, Italian seasoning, smoked paprika, salt, and black pepper. Stir for 1 minute, then pour in marinara sauce. Simmer for 8 to 10 minutes until thick.
Reduce the heat to low and stir in heavy cream, Parmesan, and half the mozzarella until the sauce becomes rich and creamy.
In the pasta pot, melt butter and cook the remaining garlic for 30 seconds. Add the fettuccine and toss in the garlic butter, then fold in the creamy beef Bolognese. Add a splash of reserved pasta water if needed.
Sprinkle the remaining mozzarella over the pasta, cover for 1 to 2 minutes until melted, then finish with fresh chives and parsley. Serve hot with plenty of creamy beef sauce.