Heat Things Up: Preheat your oven to 350°F (175°C). Prepare a 9×13 baking dish with parchment paper or a light coating of grease.
Whisk the Special Sauce: In a small bowl, combine all the sauce ingredients until smooth. Cover and let it chill in the fridge.
Bake the Beef: Spread the ground beef into the baking dish, pressing it firmly into a flat, edge-to-edge layer. Season with salt and pepper. Bake for 10–12 minutes until fully cooked. Drain the grease, remove the beef, and cut it into 12 squares.
Assemble: Slice your slider buns horizontally. Place the bottom halves in a clean dish. Spread a thin layer of sauce on each, top with a beef square, a slice of American cheese, and a sprinkle of diced onions.
The Buttery Topping: Place the top buns on. Brush generously with melted butter and sprinkle with sesame seeds.
The Final Melt: Bake uncovered for 8–10 minutes until the cheese is gooey and the buns are golden.
The Fresh Finish: Once out of the oven, lift the top buns and add the shredded lettuce and pickles. Replace the tops and serve immediately while hot!