Begin by bringing a large pot of salted water to a rolling boil. Add medium pasta shells and cook for 7-9 minutes until just tender but still firm to the bite (al dente). Drain the pasta and set it aside, allowing residual heat to keep it warm while you prepare the rest of the creamy beef and shells.

In a large skillet over medium heat, warm 1 tablespoon of olive oil. Add 1 pound of ground beef, breaking it apart with a spatula, and cook for about 3-5 minutes until browned. Ensure the beef is no longer pink, and drain any excess grease for a cleaner flavor.

Add one diced sweet onion into the skillet, stirring it with the browned ground beef. Cook for approximately 2 minutes until the onion softens and becomes translucent. Then incorporate 2 minced garlic cloves, cooking for an additional 30 seconds until fragrant.

Sprinkle 2 tablespoons of all-purpose flour over the beef and vegetable mixture, stirring to coat evenly. Gradually whisk in 1 cup of beef stock and 1 cup of marinara sauce, ensuring there are no lumps. Allow this mixture to simmer for about 6-8 minutes, stirring occasionally until it thickens.

Once the sauce has thickened, gently fold in the drained pasta shells, making sure they're well-coated with the creamy sauce. Then, stir in ½ cup of heavy cream and ½ cup of sour cream, mixing until everything is beautifully combined and creamy.

Turn off the heat and fold 1 cup of shredded cheddar cheese into the mixture, stirring until it melts into the flavors. Serve the creamy beef and shells immediately, garnished with fresh parsley if desired.