01 - Bring a large pot of generously 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 into uniform crumbles. Cook until deeply browned and no pink remains, approximately 5 to 7 minutes. Drain excess rendered fat if desired.
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 brown.
04 - Reduce heat to medium. Add unsalted butter to the skillet, allowing it to melt gently while stirring to emulsify with the beef drippings and aromatics into a cohesive, glossy sauce.
05 - Transfer the drained pasta directly into the skillet. Toss vigorously to coat every strand in the garlic butter mixture. Splash in reserved pasta water as needed, a little at a time, to achieve a silky, clinging sauce.
06 - Remove from heat. Stir in grated Parmesan cheese and chopped fresh parsley. Season with salt, freshly ground black pepper, and red pepper flakes if using. Taste and adjust seasoning to your preference.
07 - Divide among warmed plates or bowls. Garnish with additional Parmesan and a sprinkle of fresh parsley. Serve immediately while hot.