In a medium skillet over medium-high heat, crumble and brown 1 pound of ground beef for about 5–7 minutes until no longer pink. Add in one chopped onion and two minced garlic cloves, stirring occasionally until the onion turns translucent, which should take about 3–4 minutes.

Sprinkle salt and pepper to taste over the beef, then stir in 1 tablespoon of Worcestershire sauce and ½ cup of beef broth. Allow the mixture to simmer on medium-low heat for around 5–6 minutes until it thickens.

While the beef mixture simmers, butter one side of each slice of sourdough bread. Preheat a non-stick skillet or griddle over medium heat.

Place two slices of the bread, butter-side down, on the skillet. Layer with shredded cheddar, a hearty spoonful of beef mixture, and sprinkle with mozzarella cheese. Top with remaining slices of bread, butter-side up.

Cook the sandwiches for 3–4 minutes per side, pressing gently with a spatula to help the cheese melt. Look for a golden-brown crust.

Remove the sandwiches from the skillet and let them rest for a minute. Slice in half and garnish with freshly chopped parsley if desired.