Bring a large pot of salted water to a boil. Add pasta and cook for 8 minutes. The pasta should be cooked through but just under al dente. Drain and set aside.
In a heavy-bottomed pot, heat olive oil over medium heat. Add ground beef and break it up with the back of a spoon while it browns. Once cooked through, transfer to a bowl. Set aside.
Add onions to the pot and cook, stirring, until they begin to soften, about 3 minutes. Mix in garlic and Italian seasoning. Cook and stir for 30 seconds. Sprinkle flour on top and stir. The mixture will be dry.
Slowly whisk in beef broth, then add tomato sauce. Whisk until smooth. Taste and season with salt and pepper. Bring sauce to a boil, reduce heat, and simmer, stirring frequently, until reduced and slightly thickened, 8 to 10 minutes.
Add beef, pasta shells, and half-and-half to sauce and mix until well combined. Stir in cheese until completely melted.
Serve in wide bowls with parsley and Parmesan cheese for garnish.
