In a large pot or Dutch oven, melt the butter over medium-high heat. Add the ground beef and diced onion, cooking until the beef is browned. Break it apart with a spoon as it cooks. Drain any excess grease.

Add the minced garlic to the pot and cook for another minute until fragrant, stirring constantly.

Pour in the beef broth, using a spoon to scrape up any flavorful browned bits from the bottom of the pot. Stir in the heavy cream and bring the mixture to a gentle simmer.

Add the fettuccine to the pot. Press it down gently to submerge it in the liquid as much as possible. Reduce the heat to maintain a simmer, cover, and cook for 15-20 minutes, or until the pasta is al dente. Stir every few minutes to prevent sticking.

Once the pasta is cooked through and most of the liquid is absorbed, remove the pot from the heat. Stir in the grated Parmesan cheese and fresh parsley. The sauce will thicken as it cools slightly. Season with salt and pepper to taste, and serve immediately.