In a medium bowl, combine ground beef, beaten egg, Worcestershire sauce, brown mustard, and melted unsalted butter. Add bread crumbs, freshly grated Parmesan cheese, minced garlic, and finely chopped onions. Mix gently until just combined.

Divide the mixture into four equal portions and shape each into a patty about ¾ inch thick. Make an indentation in the center of each patty. Refrigerate for 20 minutes.

Preheat the grill to medium-high heat. Season the tops of each patty with chosen seasoning.

Grill the patties for 4-5 minutes on each side or until juices bubble on top. Ensure the internal temperature reaches 160°F.

In the last few minutes of cooking, add a slice of cheese to each patty and let it melt. Prepare burger buns and toppings while the cheese melts.

Remove patties from the grill and let them rest for a few minutes. Serve on toasted buns with favorite condiments.