Preheat the oven to 375°F (190°C). Line a 9x9-inch or 9x13-inch baking pan with aluminum foil and spray lightly with cooking spray.
Using a large serrated knife, slice the connected package of rolls horizontally so you have one 'slab' of bottoms and one 'slab' of tops; do not separate individual rolls. Place the bottom slab in the prepared pan.
In a large bowl, combine the cooked chicken and 1/2 cup ranch dressing; stir until the chicken is evenly coated.
Spread the ranch-coated chicken evenly over the bottom slab of rolls, then layer the sliced Swiss cheese evenly over the chicken.
Place the top slab of rolls over the cheese and set aside.
In a microwave-safe bowl, melt the 1/2 cup butter on high for about 1 minute. Whisk in the Dijon mustard, poppy seeds, dried minced onion, Worcestershire sauce, salt, and pepper until combined.
Slowly pour the butter mixture over the top of the rolls and use a spatula to spread it so the tops are evenly coated. Some of the mixture may pool around the base of the rolls.
Cover the pan with foil and let the assembled sliders stand at room temperature for 5 to 10 minutes to allow flavors to meld.
Bake covered for about 16 minutes, or until the cheese has melted. Uncover and bake an additional 4 to 5 minutes, or until the tops are lightly browned and heated through.
Remove from the oven, slice into individual sliders, and serve warm.
