In a large skillet or Dutch oven over medium-high heat, brown the ground beef, breaking it up with a wooden spoon. Cook until no longer pink, about 5-7 minutes. Drain off any excess grease.

Add the chopped yellow onion to the skillet with the beef and cook until softened, about 3-4 minutes. Add the minced garlic and cook for another minute until fragrant.

Stir in the beef broth, diced tomatoes (undrained), uncooked elbow macaroni, ketchup, yellow mustard, Worcestershire sauce, salt, and black pepper. Bring the mixture to a boil.

Once boiling, reduce the heat to medium-low, cover the skillet, and simmer for 10-12 minutes, or until the pasta is al dente and most of the liquid has been absorbed. Stir occasionally to prevent sticking.

While the pasta simmers, in a separate small bowl, whisk together the whole milk and all-purpose flour until smooth. This creates a slurry.

Once the pasta is cooked, remove the skillet from the heat. Pour the milk-flour slurry into the pasta mixture. Stir well.

Add the shredded cheddar cheese and torn American cheese slices to the skillet. Stir continuously until all the cheese is melted and the sauce is smooth and creamy. If the sauce is too thick, add a splash of milk or broth.

Taste and adjust seasonings if necessary. Ladle the Classic Cheeseburger Skillet Pasta into bowls. Garnish generously with dill pickle relish (or chopped pickles) and fresh parsley, if desired. Serve immediately.