Preheat the oven to 400°F. Grease a baking dish. *See Note
In a large pot of salted water, cook the pasta according to the package directions. Drain and set aside.
12 ounces dry elbow macaroni
While the pasta cooks, cook the ground beef and onion over medium heat in a large skillet, breaking up the beef with a spoon until the beef is no longer pink, and the onion is translucent.
1½ pounds ground beef, 1 small yellow onion
Add in the tomato paste, garlic, Italian seasoning, chili powder, salt and pepper. Cook, stirring, for 1-2 minutes until the spices are fragrant and the tomato paste darkens in color.
2 tablespoons tomato paste, 3 garlic cloves, 2 teaspoons Italian seasoning, 1 teaspoon chili powder, 1 teaspoon salt, ½ teaspoon ground black pepper
Add in the pasta, heavy cream, beef broth, 1 cup cheddar cheese, and 1 cup mozzarella cheese.
1½ cups heavy cream, ¼ cup beef broth, 2 cups shredded cheddar cheese, 2 cups shredded mozzarella cheese
Transfer to a baking dish (or leave in your oven safe skillet) and top with the remaining cheese. Cover and bake for 15 minutes. Uncover and bake, or broil for a darker crust, for 5 more minutes.
2 cups shredded cheddar cheese, 2 cups shredded mozzarella cheese
Let cool for 10 minutes, garnish with green onions, and serve.
1 teaspoon sliced green onion