01 - Bring a large pot of generously salted water to a rolling boil. Add the pasta and cook according to package directions until al dente. Before draining, reserve 1/2 cup of the starchy pasta water. Drain the pasta and set aside.
02 - Heat olive oil in a large skillet over medium-high heat. Add the ground beef, breaking it apart with a wooden spoon into bite-sized crumbles. Cook until thoroughly browned and no pink remains, approximately 5 to 7 minutes. Drain any excess rendered fat if necessary.
03 - Add the finely chopped onion to the browned beef and sauté until soft and translucent, about 3 minutes. Stir in the minced garlic and continue cooking for 1 minute until fragrant, being careful not to let the garlic brown.
04 - Reduce the heat to medium. Add the butter to the skillet, allowing it to melt gently while stirring to coat the beef and aromatics evenly in the rich garlic butter base.
05 - Transfer the drained pasta directly into the skillet. Toss vigorously to coat every strand in the garlic butter mixture. Splash in the reserved pasta water as needed, a little at a time, to create a silky, glossy sauce that clings to the pasta.
06 - Stir in the grated Parmesan cheese, chopped parsley, salt, black pepper, and crushed red pepper flakes if using. Toss until the cheese melts and evenly coats the pasta. Taste and adjust the seasoning to your preference.
07 - Serve immediately while hot, garnished with additional Parmesan and a sprinkle of fresh parsley if desired.