Place a piece of parchment paper on the bottom of a 9x13 baking dish or 10x15 jelly roll pan. Dice the onion a sprinkle over the parchment paper.
Mix the onion soup mix into the ground beef and spread the meat mixture out over the chopped onion in the baking dish to make a large patty pressed to the edge of the pan. (The meat will shrink as it cooks)
Bake the hamburger meat at 400 degrees for 20 minutes.
While the meat is cooking, prepare the rolls by cutting them in half lengthwise and placing the bottom buns on a separate baking sheet or baking dish.
Remove the hamburger patty from the oven and reduce the heat to 350 degrees. Use pat with a paper towel to remove excess grease. Use two spatulas to lift the meat off the parchment paper and place on the bun bottoms.
Place the cheese slices over the top of the meat, followed by a pickle slice over each bun bottom. Place the bun tops over the pickles and brush the tops with melted butter.
Bake for 15 more minutes until the rolls are toasted and the cheese has melted. Allow the sliders to rest for 5 minutes, before cutting into individual sliders and serve!