Heat olive oil in a large deep skillet or pot over medium-high heat.
Add the ground beef and cook, breaking it up with a spoon, until browned and cooked through, about 5-7 minutes.
Add the chopped onion and minced garlic to the beef. Cook for 3-4 minutes until the onion is softened.
Stir in the dried Italian seasoning, salt, and black pepper.
Pour in the beef broth, water, and undrained diced tomatoes. Stir to combine.
Add the pasta shells to the pot and bring the mixture to a boil.
Reduce heat to medium-low, cover, and simmer for 12-15 minutes, stirring occasionally, until the pasta is tender and most of the liquid is absorbed.
Stir in the heavy cream, shredded mozzarella, and grated Parmesan cheese until the cheese is melted and the sauce is creamy.
Remove from heat and sprinkle with chopped fresh parsley before serving.