Stir together mayonnaise, ketchup, yellow mustard, and pickle brine in a small bowl. Refrigerate while you prep the beef.
Combine ground beef with salt, pepper, garlic powder, onion powder, and Worcestershire sauce in a large bowl. Mix with your hands for 20 seconds, just until combined.
Press beef into a flat rectangle roughly 25cm x 20cm on a parchment-lined baking sheet. Score into 12 equal portions without cutting all the way through.
Heat a large cast iron skillet over medium-high heat with a light coating of neutral oil. Cook beef sheet 5-6 minutes until deep brown on the bottom. Flip as one piece and cook 3-4 minutes until internal temperature reaches 71°C (160°F).
Lay cheese slices over the hot beef, overlapping slightly. Cover with a lid or foil and wait 60-90 seconds until fully melted.
Brush slider bun cut sides with melted butter. Toast cut-side down in a dry skillet for 60-90 seconds until golden.
Cut beef along scored lines into 12 patties. Spread burger sauce on bottom buns. Top each with a patty, a pinch of onion, and two pickle slices. Close with top buns.
Optional: Arrange assembled sliders in a baking dish, brush tops with remaining butter, and bake at 175°C (350°F) for 5-7 minutes.
