Preheat oven to 350°F and spray a 9x13 baking dish or sheet pan with nonstick spray.
Combine all Big Mac sauce ingredients in a small bowl, mix well, and set aside.
Cook ground beef, chopped onions, salt, and pepper in a large skillet over medium heat until beef is no longer pink and onions are soft; drain excess fat.
Place the bottom halves of the slider buns in the prepared baking dish.
Spread a thin layer of Big Mac sauce over the bottom buns.
Evenly top the sauce with the cooked beef mixture.
Place American cheese slices over the beef to cover it as evenly as possible.
Add dill pickle slices evenly over the cheese layer.
Bake sliders for 8-10 minutes until cheese starts to melt.
Remove from oven, add shredded lettuce and additional Big Mac sauce if desired.
Top with slider bun tops; brush buns with melted butter and sprinkle with sesame seeds.
Cover with foil and bake for an additional 3-4 minutes to warm completely and melt cheese fully.
Remove from oven and serve warm.