Shred the cheddar cheese and allow it to come to room temperature.
Season the ground beef with salt and pepper. Cook and crumble it with the diced onions in a large pot over medium heat. Drain any excess grease.
Add the minced garlic and cook for 1 minute until fragrant.
Stir in the Worcestershire sauce and tomato paste until fully combined.
Pour in the beef broth and add the uncooked elbow macaroni. Stir well and bring to a gentle simmer.
Cover the pot and cook for 5 minutes. Stir, re-cover, and cook for another 5 minutes until the pasta is just al dente.
Lower the heat. Stir in the sour cream and half the milk. Add additional milk if needed for creaminess.
Gradually sprinkle in 2 ½ cups of the shredded cheddar cheese, stirring until melted and smooth.
Top the casserole with the remaining cheese. Cover and cook for another 2–3 minutes until the cheese is melted.
Garnish with chopped parsley before serving.