01 - Bring a large pot of salted water to a rolling boil. Cook spaghetti or linguine according to package directions until al dente. Reserve 1/2 cup of starchy pasta water before draining. Set the drained pasta aside.
02 - Melt 2 tablespoons of butter in a large skillet over medium heat. Add the ground beef, breaking it apart with a wooden spoon. Cook until fully browned and no pink remains, seasoning with salt and black pepper. Transfer the beef to a plate and drain any excess fat from the skillet.
03 - In the same skillet, add the remaining 2 tablespoons of butter. Sauté the chopped onion for about 2 minutes until soft and translucent. Stir in the minced garlic and cook for 1 minute until fragrant, being careful not to let it brown.
04 - Return the browned beef to the skillet. Pour in the beef broth and bring to a gentle simmer. Add the crushed red pepper flakes if desired. Let the mixture simmer for 2 to 3 minutes so the flavors meld together.
05 - Add the cooked pasta directly to the skillet and toss thoroughly to coat every strand in the garlic butter sauce. If the mixture seems too dry, gradually stir in small amounts of the reserved pasta water until a silky, glossy consistency is achieved.
06 - Remove the skillet from heat. Stir in the grated Parmesan cheese and chopped fresh parsley, tossing until the cheese melts and evenly coats the pasta.
07 - Serve immediately in warmed bowls. Offer extra Parmesan cheese and lemon wedges on the side for garnish.