Preheat your oven to 400°F and lightly spray a 9x13 inch baking dish with cooking spray. While the oven heats, slice your ham and turkey into thin, even pieces for easier layering, and slice your Swiss cheese into thin slices as well. In a small bowl, whisk together the Dijon mustard, 3 tablespoons of melted butter, and salt to create your seasoning base. I like to let this mixture cool slightly so it doesn't dry out the roll's cut sides when spread.

Carefully slice your Hawaiian roll package horizontally into two flat slabs—one top and one bottom. Place the bottom slab cut-side up in your prepared baking dish. Spread half of the mustard-butter mixture evenly over the cut side of the bottom roll slab. Layer half of the cheese slices over the mustard base, then layer all of the ham and turkey in an even distribution, and finally top with the remaining cheese slices. Spread the remaining mustard-butter mixture over the top surface of the cheese layer.

Place the top roll slab cut-side down onto the cheese and meat layer, creating a sandwich. In a small bowl, whisk together the egg and the 4 tablespoons of Kerrygold butter (or your preferred butter for a richer, more golden crust). Brush this egg-butter mixture generously over the top and sides of the assembled rolls. Sprinkle the poppy seeds evenly over the egg wash—this adds both flavor and a professional appearance.

Cover the baking dish tightly with aluminum foil and bake for 20 minutes—the foil traps steam to keep the rolls moist and helps the cheese melt evenly. Remove the foil and continue baking for 5-10 minutes until the top is golden brown and the cheese is bubbling at the edges. Watch carefully during this final stage, as ovens vary and you want a beautiful golden crust without burning the poppy seeds.

Remove the baking dish from the oven and let the sliders rest for 5 minutes—this allows the cheese to set slightly, making them easier to slice cleanly without the filling spilling out. Using a sharp serrated knife, carefully slice between each roll to create individual sliders. I find wiping the knife between cuts helps prevent the melted cheese from dragging.

Dust the warm sliders generously with sifted powdered sugar, creating a light, even coating across the top of each sandwich. Serve immediately with the raspberry jam on the side for dipping or spreading. The contrast between the sweet powdered sugar, warm cheese, savory meats, and tart jam is what makes Monte Cristo sandwiches so special.