01 - Bring a large pot of generously salted water to a rolling boil. Cook the 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 the ground beef and cook, breaking it apart with a wooden spoon, until thoroughly browned and no pink remains, 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.
04 - Reduce the heat to medium. Add the unsalted butter to the skillet, allowing it to melt while stirring to coat the beef and aromatics evenly in the rich garlic butter mixture.
05 - Transfer the drained pasta directly into the skillet. Toss vigorously to coat every strand in the garlic butter sauce, splashing in reserved pasta water as needed to achieve a silky, glossy consistency.
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.
07 - Plate immediately while hot. Garnish with additional Parmesan cheese and a sprinkle of fresh parsley if desired. Pair with a crisp green salad and garlic bread for a complete meal.