Preheat oven to 375 degrees.
Line a large rimmed baking dish with parchment paper and press the ground beef into a flat rectangle patty, at least 14x10" in size (it will shrink as it cooks). Season with salt and black pepper, then place it in the oven for 10-15 minutes, or until the internal temperature reaches 150 degrees.
Prepare the Big Mac sauce while the patty bakes by whisking all of the sauce ingredients together in a bowl.
Slice the slider buns in half horizontally, and place the baked patty on the buns in a large baking dish, discarding the grease. Place the top buns on the patty and brush with melted butter, and sprinkle sesame seeds over the top. Bake the sliders in the oven for 5 minutes to allow the buns to warm and the patty to cook further.
Remove the sliders from the oven and take off the upper bun. Add American cheese slices over the hot patty, then top with minced onion, pickle slices, and shredded lettuce. Spread the Big Mac sauce over the bottom side of the upper buns, then place the buns on top of the sliders.
Slice the sliders into individual servings and serve immediately.