Fill a large pot with salted water and bring it to a rolling boil. Add medium pasta shells and cook until just under al dente, about 8 minutes. Drain and set aside, reserving a cup of pasta water.

In a large skillet over medium-high heat, add olive oil and let it heat. Add ground beef, breaking it apart while cooking until browned, 3-5 minutes. Drain excess fat.

In the same skillet, add diced sweet onion and sauté for about 2 minutes until soft. Stir in minced garlic and cook for another minute until fragrant.

Sprinkle all-purpose flour over the beef and onions, whisking for about a minute. Gradually mix in beef stock and marinara sauce, stirring continuously. Add Italian seasoning, dried parsley, dried oregano, and smoked paprika. Bring to a boil, then reduce heat and simmer for 6-8 minutes until thickened.

Fold in the cooked pasta and browned beef until well-coated. Slowly drizzle in heavy cream and mix in sour cream for tanginess.

Turn off the heat and fold in grated cheddar cheese until melted. Serve hot, garnished with fresh parsley if desired.