In a skillet over medium-high heat, cook the ground beef until browned, breaking it apart as it cooks. Add in the finely chopped onion and minced garlic, sautéing until the onion turns translucent and fragrant.
Season the beef mixture with salt and pepper to taste. Stir in Worcestershire sauce and beef broth, allowing everything to simmer until the liquid reduces slightly, enhancing flavors.
Butter one side of each slice of bread generously with softened unsalted butter. Preheat your skillet or griddle over medium heat so it's hot enough for grilling.
Place two slices of bread on the skillet with the butter side down. Layer half of your shredded cheddar followed by the beef mixture and then top with mozzarella cheese before placing another slice of bread on top, butter side up.
Grill each side of the sandwich for about 3–4 minutes until they are golden brown and crispy while ensuring that the cheese melts beautifully inside. Flip carefully using a spatula to maintain structure.
Once grilled perfectly, remove from heat, slice in half, and serve warm. For an added touch, garnish with freshly chopped parsley if desired.