Pasta Preparation: Commence by cooking the pasta shells according to the instructions on the package. Utilize a large pot of boiling salted water for this purpose. Once cooked, drain the pasta well and set it aside.
Cooking the Beef: In a large skillet, heat the olive oil over medium-high heat. Add the ground beef to the skillet and cook until it is thoroughly browned, which should take approximately 3-5 minutes. Make sure to break the beef apart with a wooden spoon during cooking. After the beef is cooked, drain any excess fat and set the beef aside.
Sautéing Onion and Garlic: In the same skillet used for the beef, add the diced onion. Cook for 2 minutes, ensuring that you stir frequently. Following the onions, add the minced garlic and continue to cook until the garlic becomes fragrant, which should take about 1 minute.
Creating the Roux: Add the flour to the skillet and whisk it for about 1 minute, until it is lightly browned. This step is crucial for thickening the sauce.
Adding Liquids and Seasonings: Gradually whisk in the beef stock into the skillet, ensuring that you stir well to combine the ingredients. Subsequently, add the marinara sauce to the mixture. Incorporate the Italian seasoning, dried parsley, oregano, and paprika by stirring them in.
Simmering the Sauce: Bring the sauce to a boil, then reduce the heat. Allow the sauce to simmer, stirring occasionally, until it has reduced and thickened slightly. This should take approximately 6-8 minutes.
Combining Pasta and Beef: Add the previously cooked pasta to the skillet, along with the set-aside beef. Stir to ensure that the pasta and beef are well integrated with the sauce.
Incorporating Dairy: Stir in the heavy cream and continue to cook for about 1-2 minutes until the mixture is heated through. At this point, taste the sauce and adjust the seasoning with salt and pepper as needed. Next, add the sour cream and stir.
Adding Cheese: Finally, fold in the grated cheddar cheese and continue to cook until the cheese is completely melted, which should take an additional 1-2 minutes.
Serving: Serve the Creamy Beef and Shells immediately. Optionally, it can be garnished with parsley for an enhanced presentation.