Cook the pasta shells in a large pot of boiling salted water according to package directions until al dente, usually 8-9 minutes. Drain well in a colander and set aside.

Heat olive oil in a large skillet over medium-high heat. Add the ground beef and cook for 3-5 minutes, breaking it apart with a wooden spoon until browned and no pink remains. Drain most of the excess fat, leaving about 1-2 tablespoons in the pan, and set the cooked beef aside in a bowl.

Add the diced onion to the same skillet and cook for 2 minutes, stirring frequently until translucent. Add the minced garlic and cook for another minute until fragrant.

Sprinkle the flour over the onion and garlic mixture and whisk continuously for about 1 minute until the flour turns lightly golden.

Gradually whisk in the beef stock, stirring constantly to prevent lumps. Pour in the marinara sauce, then add the Italian seasoning, dried parsley, oregano, and smoked paprika. Stir everything together until well combined.

Bring the mixture to a boil, then reduce heat to medium-low and let it simmer for 6-8 minutes, stirring occasionally, until the sauce reduces and thickens noticeably.

Stir in the cooked pasta shells and return the browned beef to the skillet. Pour in the heavy cream and stir for 1-2 minutes until heated through. Taste and adjust with salt and pepper as needed.

Stir in the sour cream until completely incorporated. Finally, fold in the freshly grated cheddar cheese and stir for 1-2 minutes until melted and creamy. Serve immediately, garnished with fresh parsley if desired.