01 - Bring a large pot of generously salted water to a rolling boil. Add the spaghetti or fettuccine and cook according to package directions until al dente. Reserve 1/2 cup of starchy pasta water before draining in a colander.
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 deeply browned and no longer pink throughout, 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 softened and translucent, about 3 minutes. Stir in the minced garlic and continue cooking for 1 minute until fragrant, taking care not to let it brown.
04 - Reduce heat to medium. Add the unsalted butter to the skillet, allowing it to melt gently while stirring to emulsify with the beef and aromatics, creating 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, clinging sauce.
06 - Stir in the grated Parmesan cheese, chopped fresh parsley, salt, black pepper, and crushed red pepper flakes if using. Taste and adjust the seasoning to your preference before serving.
07 - Serve immediately in warm bowls, garnished with additional Parmesan and a scattering of fresh parsley if desired.