01 - Bring a large pot of salted water to a rolling boil. Cook spaghetti or fettuccine according to package directions until al dente. Reserve 1/2 cup of starchy pasta water before draining through a colander.
02 - Heat olive oil in a large skillet over medium-high heat. Add ground beef, breaking it apart with a wooden spoon, and cook until deeply browned and no pink remains, approximately 5 to 7 minutes. Drain excess rendered fat if necessary.
03 - Add the finely chopped onion to the browned beef and sauté until softened and translucent, about 3 minutes. Stir in the minced garlic and cook for 1 additional minute until fragrant, being careful not to let it scorch.
04 - Reduce heat to medium. Add unsalted butter to the skillet, allowing it to melt gently while stirring to emulsify with the beef and aromatics, forming a rich, glossy base.
05 - Transfer the drained pasta directly into the skillet. Toss vigorously to coat every strand in the garlic butter mixture, splashing in reserved pasta water as needed to achieve a silky, clingy sauce.
06 - Stir in grated Parmesan cheese, chopped parsley, salt, black pepper, and crushed red pepper flakes if using. Taste and adjust seasoning to your preference before serving.
07 - Divide among warmed plates or bowls immediately. Garnish with additional Parmesan and a sprinkle of fresh parsley for a polished presentation.