Cook the pasta: Bring a large pot of salted water to a boil. Add the pasta shells and cook according to the package instructions until al dente. This usually takes around 8 to 10 minutes. Drain the pasta and set it aside while you prepare the sauce.
Sauté the onion and garlic: Heat the olive oil in a large skillet over medium heat. Add the chopped onion and cook for about 3 to 4 minutes until it softens and becomes slightly translucent. Stir in the minced garlic and cook for another 30 seconds until fragrant.
Brown the beef: Add the ground beef to the skillet. Break it up with a spoon as it cooks and allow it to brown evenly. This usually takes about 5 to 7 minutes. If there is excess grease, you can carefully drain a little of it.
Build the creamy sauce: Pour in the marinara sauce, beef broth, and heavy cream. Stir everything together and let the mixture simmer gently for about 5 minutes.
Combine the pasta and cheese: Add the cooked pasta shells to the skillet and stir until they are coated in the sauce. Sprinkle the shredded cheddar cheese over the pasta and continue stirring until the cheese melts completely.
Season and serve: Taste the dish and season with salt and pepper as needed. Serve the pasta hot while the sauce is still creamy and the cheese is perfectly melted.