1. In a mixing bowl, combine ground chuck with Worcestershire sauce, salt, and garlic powder. Form into two thin, wide patties roughly the size of your bread slices.

2. Heat a cast iron skillet over medium heat for 5 minutes. Add 1 tbsp butter and the sliced onions. Cook slowly for 25 minutes, stirring occasionally, until deeply caramelized. Finish with balsamic vinegar if desired. Set aside.

3. Wipe out the skillet and add a bit more butter if needed. Cook the beef patties 3–4 minutes per side until browned and cooked through. Remove and set aside.

4. Butter one side of each bread slice. Layer the sandwich: one slice bread (buttered side out), cheddar, patty, caramelized onions, mozzarella, top bread slice (buttered side out).

5. Grill sandwich in the skillet over medium heat, pressing with a spatula or another skillet. Cook 3–4 minutes per side until bread is golden and cheese is melted.

6. Let the sandwich rest for 1 minute after cooking, then slice and serve.