Bring a large pot of salted water to a rolling boil over high heat. Add the rotini pasta and cook until al dente, usually about 8-10 minutes. Drain the pasta in a colander, reserving ½ cup of the pasta water. Set aside.

In a large skillet, heat over medium heat and add the ground beef. Cook for about 5-7 minutes, stirring frequently, until browned. Drain any excess grease.

Add the finely chopped onion to the skillet with the beef. Sauté for 3-4 minutes until softened. Stir in the minced garlic and cook for an additional 30 seconds.

Pour in the heavy cream and beef broth. Stir to combine and bring to a gentle simmer. Allow the sauce to simmer for 3-5 minutes until thickened.

Reduce heat to low, stir in the shredded cheddar and mozzarella cheeses until melted. If using diced tomatoes, fold them in. Season with salt and black pepper.

Add the cooked rotini pasta into the skillet with the creamy sauce. Toss gently until well coated. If too thick, add reserved pasta water to reach desired consistency.

For an extra cheesy finish, sprinkle additional cheese on top. Cover for 1-2 minutes to let cheese melt. Garnish with freshly chopped parsley and serve hot.