Begin by boiling a pot of salted water and cook the pasta shells until al dente. Drain them well, making sure no extra water remains. Set them aside while you prepare the sauce.
Heat olive oil in a large skillet over medium-high heat and brown the ground beef, breaking it up as it cooks. Drain excess fat, then add diced onion and sauté until softened. Add garlic and Italian seasoning, stirring until fragrant.
Sprinkle flour over the mixture and stir until lightly toasted. Gradually whisk in the beef broth and tomato sauce until smooth. Bring the sauce to a gentle boil, then reduce heat and simmer until slightly thickened. Stir in the heavy cream, cooked pasta, and cheddar cheese. Mix until the cheese melts and everything is beautifully creamy. Season with salt and pepper and serve warm.