Prepare the special sauce: In a small mixing bowl, whisk together mayonnaise, French dressing, sweet pickle relish, minced onion, white vinegar, sugar, and salt. Set aside.

Cook the beef: In a large skillet over medium heat, cook ground beef and diced onion until the beef is browned and the onions are translucent. Season with seasoned salt and pepper. Drain excess grease.

Assemble the sliders: Preheat the oven to 350°F (175°C). Slice the slider buns horizontally, keeping them connected. Place the bottom half in a 9×13-inch baking dish. Spread half of the special sauce over the bottom buns. Evenly distribute the cooked beef mixture over the sauce, then layer with cheddar cheese slices. Add sliced pickles on top. Place the top half of the buns over the fillings.

Add toppings: Brush the melted butter over the top of the buns and sprinkle with sesame seeds, if desired.

Bake: Cover the dish with foil and bake for 20 minutes, or until the cheese is melted and the sliders are heated through.

Serve: Remove from the oven, uncover, and let cool slightly. Cut into individual sliders, garnish with shredded lettuce, and serve warm.