Heat a large Dutch oven over medium heat and add the olive oil. When hot, add the diced onion and cook for 3 to 4 minutes, stirring occasionally, until softened and lightly translucent.

Stir in the minced garlic and cook for 30 seconds, being careful not to let it brown.

Add the ground beef, salt, and black pepper to the Dutch oven. Cook, breaking the meat up with a wooden spoon, for 5 to 7 minutes, until fully browned and no pink remains. If there's excess grease, spoon off most of it, leaving a thin layer for flavor.

Turn off the heat and pour the uncooked pasta shells directly over the browned beef. Do not stir; you want the shells to sit on top of the meat.

In a bowl, whisk together the beef broth, heavy cream, tomato paste, and Italian seasoning until smooth. Whisk in 1 cup of shredded cheddar cheese.

Return the Dutch oven to medium heat and slowly pour the creamy mixture over the pasta shells and beef. Use the back of a spoon to press any exposed shells down.

Bring the mixture to a gentle simmer, then reduce the heat to low. Cover and cook for 15 to 18 minutes, stirring once or twice, until the pasta is tender and the sauce has thickened.

Remove the lid and check the consistency. If it's too thick, stir in a splash of beef broth or water. If it's too thin, simmer uncovered for 2 to 3 more minutes.

Turn off the heat and stir in the remaining 1/2 cup shredded cheddar cheese until melted. Let it rest for 3 to 5 minutes to help the sauce cling to the pasta.

Serve hot, sprinkled with grated Parmesan and fresh parsley if desired.