In a large skillet, add the ground beef and onion over medium-high heat and brown the ground beef, breaking the meat apart well as it cooks.
Drain grease, if needed, then add bouillon cube, water, brown sugar, apple cider vinegar, and Worcestershire sauce. Bring to a boil, then reduce heat and let simmer until liquid has evaporated.
Place bottoms of the buns in a baking dish, and top with beef mixture. Place pickles on top, followed by cheese slices, and top with top of buns.
In a small bowl, whisk together melted butter, brown sugar, Dijon mustard, and Worcestershire sauce and brush over buns.
Cover baking dish with foil and bake 25 minutes. Enjoy!