01 - Bring a large pot of salted water to a rolling boil. Add the pasta and cook according to package directions until al dente. Reserve 1/2 cup of the starchy pasta water before draining through 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 thoroughly browned with no pink remaining, 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 cook for an additional minute until fragrant.
04 - Reduce the heat to medium. Add the butter to the skillet, allowing it to melt completely while stirring to coat the beef and aromatics evenly in the rich garlic butter.
05 - Transfer the drained pasta directly into the skillet. Toss vigorously to coat every strand in the garlic butter mixture, adding small splashes of the reserved pasta water as needed to achieve a silky, cohesive sauce.
06 - Stir in the grated Parmesan cheese, chopped parsley, salt, black pepper, and crushed red pepper flakes if using. Taste and adjust the seasoning to your preference.
07 - Serve immediately while hot, garnished with additional Parmesan cheese and fresh parsley as desired.