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 - In a large skillet over medium heat, melt 2 tablespoons of butter. Add the ground beef, breaking it apart with a wooden spoon. Cook until thoroughly browned and no pink remains. Season with salt and black pepper to taste. Transfer the beef to a plate and drain any excess fat from the skillet if necessary.
03 - Return the same skillet to medium heat and add the remaining 2 tablespoons of butter. Sauté the finely chopped onion for about 2 minutes until translucent. Add the minced garlic and cook for 1 minute until fragrant, stirring frequently to prevent burning.
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, allowing the flavors to meld together.
05 - Add the cooked pasta to the skillet and toss thoroughly to coat every strand in the garlic butter sauce. If the mixture appears too dry, gradually stir in the reserved pasta water, a few tablespoons at a time, until the sauce reaches a silky, glossy consistency.
06 - Remove the skillet from heat. Stir in the grated Parmesan cheese and chopped fresh parsley until the cheese is melted and evenly distributed throughout the pasta.
07 - Divide the pasta among four serving bowls. Garnish with extra Parmesan and serve with lemon wedges on the side. Serve immediately while hot.