Step 1: Cook the Pasta - Cook the pasta shells according to the package instructions in a large pot of salted boiling water. Once cooked, drain the pasta and set aside.

Step 2: Sauté the Beef - In a large skillet, heat the olive oil over medium-high heat. Add the ground beef and cook it for about 3-5 minutes, breaking it up with a wooden spoon until it is browned. Drain any excess fat and set the beef aside.

Step 3: Cook the Vegetables - In the same skillet, add the diced onion. Cook for about 2 minutes, stirring frequently. Then, add the minced garlic and cook until fragrant, about 1 minute.

Step 4: Make the Sauce - Whisk in the flour and cook until it’s lightly browned, which should take about 1 minute. Gradually pour in the beef stock, stirring to combine. Next, add the marinara sauce along with the Italian seasoning, dried parsley, oregano, and smoked paprika.

Step 5: Simmer - Bring the mixture to a boil, then reduce the heat to low and let it simmer. Stir occasionally until the sauce is reduced and slightly thickened, which should take about 6-8 minutes.

Step 6: Combine Everything - After the sauce has thickened, stir in the cooked pasta and the browned beef. Then, pour in the heavy cream and heat everything through for about 1-2 minutes. Taste and season with salt and pepper as needed.

Step 7: Add the Final Touches - Finally, stir in the sour cream. Then, fold in the freshly grated cheddar cheese until melted and well combined, about 1-2 minutes.

Step 8: Serve - Serve the Creamy Beef and Shells immediately, and feel free to garnish with some chopped parsley if you desire.