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 evenly browned and no pink remains, about 5 to 7 minutes. Season generously with salt and black pepper. Transfer the beef to a plate and drain any excess fat from the skillet.
03 - Return the skillet to medium heat and add the remaining 2 tablespoons of butter. Sauté the finely chopped onion for 2 minutes until soft and translucent. Add the minced garlic and cook for 1 minute, stirring constantly, until fragrant and lightly golden.
04 - Return the browned beef to the skillet. Pour in the beef broth and bring the mixture to a gentle simmer. Add the crushed red pepper flakes if desired. Let the sauce reduce slightly for 2 to 3 minutes, allowing the flavors to meld together.
05 - Add the cooked pasta directly to the skillet. Toss vigorously to coat every strand in the garlic butter sauce. If the mixture appears 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 until the cheese melts and coats the pasta evenly. Serve immediately on warmed plates, accompanied by extra Parmesan and lemon wedges on the side.