01 - Bring a large pot of salted water to a rolling boil. Add the pasta and cook according to package directions until al dente. Reserve 1/2 cup of the starchy pasta water before draining.
02 - Heat olive oil in a large skillet over medium-high heat. Add the ground beef, breaking it apart with a wooden spoon, and cook until thoroughly browned and no pink remains, approximately 5 to 7 minutes. Drain any excess fat if necessary.
03 - Add the chopped onion to the browned beef and sauté until softened and translucent, about 3 minutes. Stir in the minced garlic and continue cooking for 1 minute until fragrant.
04 - Reduce the heat to medium. Add the butter to the skillet, allowing it to melt while stirring to coat the beef and aromatics evenly in the rich garlic butter.
05 - Transfer the drained pasta directly into the skillet. Toss vigorously to coat every strand in the garlic butter mixture, adding splashes of the reserved pasta water as needed to achieve a silky, glossy sauce.
06 - Stir in the Parmesan cheese, chopped parsley, salt, black pepper, and red pepper flakes if using. Taste and adjust the seasoning to your preference.
07 - Serve immediately while hot, garnished with additional Parmesan and fresh parsley as desired.