Preheat your oven to 350 degrees F. Line a baking sheet with aluminum foil. Set aside.
Use a serrated knife to cut the 12 Hawaiian style rolls in half horizontally, separating the tops from the bottoms. Leave the rolls attached to each other - don't pull them apart first. Set the bottom slab on the prepared baking sheet.
A serrated knife makes cutting through the entire slab much easier and keeps everything even.
In a small bowl, whisk together 1/4 cup mayonnaise and 1 1/2 tablespoons dijon mustard. Spread half of the mayo mixture over the cut side of the roll bottoms.
Lay the 8 slices honey ham and 8 slices smoked turkey over the roll bottoms so the bread is covered edge to edge. I'm usually able to put 4 slices per layer. Lay the 6 slices provolone cheese over the meats, letting the slices overlap until all the meat is covered. Add any optional toppings you're including now.
Spread the rest of the mayo mixture over the cut side of the roll tops. Carefully place the roll tops over the cheese, cut side down.
Melt 2 tablespoons unsalted butter in a small bowl. Stir in 1/2 teaspoon dried parsley and 1/4 teaspoon garlic powder until combined. Use a pastry brush to spread the butter mixture all over the tops of the sliders.
Bake the sliders for 15 minutes, or until the cheese is melted and the rolls are golden and slightly crispy.
Remove from the oven and use a spatula or sharp knife to cut the sliders apart. Serve warm.
Video
Notes
Make it ahead: Assemble up to one day in advance, cover, and refrigerate. Add the butter topping just before baking.
Double the batch easily. Make two trays (or one half sheet pan) and bake at the same time if your oven has the space.
Leftover holiday ham or turkey works great here. Just slice it thin enough that the sliders are easy to bite into.
Store leftovers in an airtight container in the refrigerator for up to 3 days. Reheat in a 325°F oven until warmed through.