Step 1: Prepare the oven. Preheat it to 350 degrees F.
Step 2: In a large skillet, place the onion and ground beef. Cook the beef over medium heat until no longer pink.
Step 3: Drain the excess grease if needed, then add the water along with the brown sugars, apple cider vinegar, Worcestershire sauce, and bouillon cube. Bring everything to a boil. Once boiling, decrease the heat and let the mixture simmer until the liquid has evaporated.
Step 4: Into a baking dish, place the bottoms of the buns and cover with the beef mixture. On top, add the pickles, and slices of cheese, and finish with the top buns.
Step 5: In a small bowl, place the melted butter, brown sugar, Dijon mustard, and Worcestershire sauce. Mix well until combined and brush the mixture on top of the buns.
Step 6: Cover the dish with foil and place it in the preheated oven. Bake for about 25 minutes.
Step 7: Remove from the oven when done and serve the sliders immediately. Enjoy!