Heat your oven to 350F and grease a 9x13 baking dish or sheet pan with nonstick spray.
Whisk together all the big mac sauce ingredients in a small bowl until well mixed, then set it aside.
Cook the ground beef with the chopped onions, salt, and pepper in a large skillet over medium heat until no longer pink and the onions turn soft, then drain the excess fat.
Arrange the bottom halves of the slider buns in the prepared dish.
Spread a thin coating of the big mac sauce on each bottom bun.
Layer the cooked beef and onion mixture evenly over the bottoms.
Cover the beef with slices of American cheese as evenly as possible.
Place the dill pickle slices on top of the cheese, spacing them so each slider gets some pickles.
Bake the sliders uncovered for 8 to 10 minutes.
Remove from the oven and add shredded lettuce and extra big mac sauce if desired.
Place the top halves of the slider buns on the sliders.
Melt the butter and brush it generously over the tops of the buns, then sprinkle with sesame seeds.
Cover the sliders with foil and bake them for another 3 to 4 minutes until warmed through and the cheese is melted.
Remove from the oven and serve immediately while warm.