Step 1: Preheat your oven to 350°F (175°C). Lightly grease a 9x13-inch baking dish.
Step 2: Using a large serrated knife, carefully slice the entire slab of Hawaiian rolls horizontally through the middle, keeping the rolls connected. Place the bottom half of the rolls into the prepared baking dish.
Step 3: Layer the ham slices evenly over the bottom half of the rolls. Follow with an even layer of cheese slices, ensuring each slider will have a good amount of melted cheese.
Step 4: Place the top half of the Hawaiian rolls back on top of the cheese and ham layers.
Step 5: In a small bowl, whisk together the melted butter, Dijon mustard, Worcestershire sauce, onion powder, garlic powder, and poppy seeds (if using). Season with salt and pepper to taste.
Step 6: Generously brush the butter mixture over the top of the rolls, ensuring to cover them thoroughly and allowing some to drip down the sides.
Step 7: Cover the baking dish tightly with aluminum foil. Bake for 15 minutes.
Step 8: Remove the foil and continue to bake for an additional 5-7 minutes, or until the tops are golden brown and the cheese is bubbly and melted. Keep a close eye on them to prevent overbrowning.
Step 9: Remove from the oven and let the sliders rest for about 5 minutes before slicing along the original roll perforations. Garnish with fresh chopped parsley or sesame seeds, if desired, and serve warm.