In a large skillet over medium heat, brown the ground beef until fully cooked, about 5–7 minutes. Drain excess fat if necessary.
Add the diced onion to the skillet and cook for an additional 2–3 minutes, until softened. Stir in the minced garlic and cook for another 30 seconds until fragrant.
If not pre-cooked, you can cook the elbow macaroni according to package instructions in a separate pot while preparing the meat mixture.
Once the onion is softened, add the beef broth, milk, ketchup, mustard, and cooked elbow macaroni to the skillet. Stir until combined.
Bring the mixture to a simmer and cook for 5 minutes, stirring occasionally. This will allow the pasta to soak up some of the flavors.
Add the shredded cheddar and mozzarella cheeses, stirring until melted and well incorporated. Season with salt and pepper to taste.
Remove from heat and let it sit for a few minutes to thicken slightly before serving.
Serve the Cheeseburger Pasta Skillet hot and top with optional toppings such as pickles, diced tomatoes, or additional cheese, if desired.