Heat a deep skillet over medium-high heat for about 2 minutes. Add the ground beef, cooking until it's well browned and no longer pink, which should take around 5-7 minutes. Use a spatula to break it up as it cooks. Drain any excess grease if needed.

Once the beef is browned, push it to the side of the skillet. Melt 2 tablespoons of butter in the cleared space, and add minced garlic and chopped onion. Sauté for about 2 minutes until the onion is translucent and fragrant. Stir in Italian seasoning and red pepper flakes.

Pour in 3 cups of beef broth and 1 cup of milk, bringing the mixture to a gentle boil. Stir in 12 ounces of uncooked pasta. Cover the skillet and reduce the heat to medium-low, letting it simmer for 12-15 minutes.

After the pasta has cooked, reduce the heat to low. Stir in 1 cup of grated Parmesan cheese along with salt and black pepper to taste. Continue stirring until the cheese melts into the sauce.

Remove the skillet from heat and garnish your One-Pot Creamy Beef and Garlic Butter Pasta with freshly chopped parsley and additional Parmesan. Serve hot.