In a skillet, cook ground beef and chopped onion over medium heat until beef is browned and onions are tender. Drain excess grease.
Stir in mustard, Worcestershire, garlic powder, salt, pepper, and beef broth. Simmer for 3–4 minutes until liquid reduces. Remove from heat.
Slice buns in half horizontally and place bottoms in a greased 9x13 dish.
Spoon beef mixture over buns, then top with cheese and pickles. Place top buns on.
Mix melted butter and onion powder, then brush over top buns.
Bake at 350°F (175°C) for 12–15 minutes until golden and heated through.
Rest 5 minutes, then slice and serve.