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.
02 - Melt 2 tablespoons of butter in a large skillet over medium heat. Add ground beef, breaking it apart with a wooden spoon, and cook until evenly browned and no pink remains. Season generously with salt and black pepper. Transfer to a plate and drain excess fat if needed.
03 - In the same skillet, add the remaining 2 tablespoons of butter. Sauté the chopped onion for 2 minutes until soft and translucent. Add minced garlic and cook for 1 minute until fragrant, stirring constantly to prevent burning.
04 - Return the browned beef to the skillet. Pour in the beef broth and bring to a gentle simmer. Stir in crushed red pepper flakes if desired. Let the liquid reduce slightly for 2 to 3 minutes.
05 - Add the drained pasta to the skillet and toss thoroughly to coat every strand in the garlic butter sauce. If the mixture seems dry, gradually add reserved pasta water, a splash at a time, until silky and well combined.
06 - Remove the skillet from heat. Stir in grated Parmesan cheese and chopped fresh parsley until the cheese melts and coats the pasta evenly. Serve immediately with extra Parmesan and lemon wedges alongside.