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. 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 deeply browned and no longer pink, approximately 5 to 7 minutes. Drain any 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 an additional minute until fragrant, being careful not to let it brown.
04 - Reduce the heat to medium. Add the unsalted butter to the skillet, allowing it to melt gently while stirring to coat the beef and aromatics evenly. The butter will create a rich, glossy base for the sauce.
05 - Transfer the drained pasta directly into the skillet. Toss vigorously to coat every strand in the garlic butter mixture. Gradually add the reserved pasta water, a splash at a time, tossing continuously until a silky, emulsified sauce clings to the pasta.
06 - Remove the skillet from heat. Stir in the grated Parmesan cheese and chopped fresh parsley until the cheese melts into the sauce. Season with salt, freshly ground black pepper, and crushed red pepper flakes if desired. Taste and adjust the seasoning to your preference.
07 - Divide the pasta among warmed plates or bowls. Garnish with an extra sprinkle of Parmesan and a scattering of fresh parsley. Serve immediately while hot.