Preheat the oven to 350 degrees F. Grease an 9x9" baking dish with cooking spray and set aside.
In a medium bowl, combine the ground beef and diced onion. Season with salt and pepper.
Press the ground beef mixture into the bottom of baking dish.
Bake or 10-15 minutes, until the beef is cooked through. It will shrink in the baking dish and there will also be a bit of grease around the large beef patty. Use a paper towel to soak up the grease.
To make the Big Mac sauce combine all sauce ingredients in a bowl and mix well to combine.
Place slider buns on a baking sheet.
Spread ~2 tablespoon of the Big Mac sauce onto each slider bottom.
Sprinkle lettuce over the sauce and then top with the burgers and pickles. You can do this two ways - you can either place the large burger patty over the buns as a whole, and then cut them into individual sliders after they are baked, or you can cut the burger patty into smaller patties and place them individually onto each slider bun bottom. Either way works!
Top the burgers with slices of American cheese and then each slider bun.
Brush the tops of the slider buns with butter and then sprinkle with sesame seeds.
Bake in the oven for 5-10 minutes, until the cheese has slightly melted.
Cut into individual sliders, serve with additional sauce, and enjoy!