In a large skillet over medium heat, brown the ground beef until fully cooked. Drain excess fat if necessary.
Add the diced onion and minced garlic to the skillet. Sauté for about 3-4 minutes until the onion is translucent.
Stir in the diced tomatoes, beef broth, Italian seasoning, salt, and black pepper. Bring the mixture to a simmer.
Add the pasta shells to the skillet, cover, and cook for about 10-12 minutes, or until the shells are al dente, stirring occasionally.
Reduce the heat to low and stir in the heavy cream, cheddar cheese, and Parmesan cheese. Cook until the cheese is melted and the sauce is creamy, about 3-5 minutes.
Serve hot, garnished with chopped parsley.