Preheat a skillet over medium heat. Add the ground beef and chopped yellow onion. Break up the beef with a spatula and brown the mixture, stirring occasionally. Once the beef is cooked through, drain any excess grease from the skillet. Season the mixture with Worcestershire sauce, garlic powder, salt, and black pepper to taste, stirring to combine. Transfer the cooked beef and onion mixture to a bowl, then wipe out or clean the skillet to prepare it for the next steps.

Place two slices of bread on a clean work surface. Sprinkle half of the shredded mozzarella cheese evenly over one slice of bread, then top with half of the cooked beef and onion mixture from Step 1. Place a second slice of bread on top to form a sandwich. Repeat this process with the remaining bread, cheese, and beef mixture to create a second sandwich. I like to gently press the sandwiches to help them hold together before grilling.

Spray the cleaned skillet with olive oil spray and heat it over medium heat. Place one assembled sandwich in the skillet and cook for 3-4 minutes on each side, or until the bread is golden brown and the cheese is melted. Carefully remove the sandwich from the skillet. Repeat the process with the second sandwich. For even crispier sandwiches, I sometimes press them gently with a spatula as they cook.