Big Mac Sauce: In a small mixing bowl, whisk together the mayonnaise, sweet pickle relish, dried minced onion, yellow mustard, white vinegar, paprika, onion powder, garlic powder, salt and ketchup, if desired. Refrigerate until ready to assemble the sliders to allow the flavors to meld.
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.
In a mixing bowl, combine the ground beef and onion soup mix. 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 patty 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. Top the bottom buns with American cheese slices.
Remove the hamburger patty from the oven and reduce the heat to 350 degrees. Pat the meat with a paper towel to remove excess grease. Use two spatulas to lift the meat off the parchment paper and place on cheese-covered bun bottoms.
Unwrap and place the remaining American Cheese slices over the meat, then spread the Big Mac sauce over cheese. Then place a pickle slice over each bun bottom. Top with shredded lettuce. Replace the bun tops over the lettuce. Brush the tops with melted butter followed by the egg wash and sprinkle with sesame seeds.
Bake for 10-15 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!