Preheat oven to 350F. Line a 9×9-inch or 9×13-inch pan with aluminum foil, spray with cooking spray; set aside.
Using a large serrated knife, slice the rolls in half so you have a top half and a bottom half; but don’t pull the rolls apart. Place the bottom half of the rolls in prepared pan.
Evenly layer the ham over the rolls.
Evenly layer the cheese.
Evenly layer the sliced turkey deli meat
Add the top half of rolls; set aside.
In a medium microwave-safe bowl, add the butter and heat on high power to melt, about 1 minute.
Add the mustard, poppy seeds, onion, salt, and pepper, and whisk to combine.
Evenly brush the butter mixture over the rolls.
Cover with aluminum foil and bake covered for about 20 minutes or until cheese has melted.
Uncover and slice into individual sliders and serve immediately, or if serving at a picnic, keep it covered until you arrive at your destination. Sliders are best warm and fresh, but may be served within an hour of heating.