Boil a large pot of salted water. Add the medium pasta shells and cook until just under al dente, about 8-10 minutes. Drain and set aside in a mixing bowl.

In a large skillet, heat 1 tablespoon of olive oil over medium heat. Add the ground beef, season with salt and pepper, and brown for 3-5 minutes. Drain excess fat.

Add the diced onion to the skillet and sauté for 2 minutes. Then, add minced garlic and stir for 30 seconds until fragrant.

Sprinkle flour over the beef and onions in the skillet, mixing well. Cook for 1-2 minutes, then whisk in beef stock and marinara sauce. Bring to boil, then simmer for 6-8 minutes until sauce thickens.

Add the drained pasta shells and sautéed beef to the sauce. Pour in heavy cream and sour cream, mixing until well blended. Heat on low for 2-3 minutes.

Remove from heat and fold in shredded cheddar cheese until melted and creamy. Serve warm, garnished with parsley if desired.