Preheat your oven to 350 degrees F. Line a baking sheet with aluminum foil.
Use a serrated knife to cut the rolls in half, separating the top from the bottom. Set the bottom half of the rolls on the prepared baking sheet.
In a small bowl, mix together the mayonnaise and mustard. Spread half of the mayo mixture over the cut side of the roll bottoms.
Lay the ham slices and then the turkey slices over the roll bottoms so all of the bread is covered. I'm usually able to put 4 slices per layer. Lay the cheese over the meats, letting the slices overlap until all the meat is covered.
Spread the rest of at mayo mixture over the cut side of the roll tops. Carefully turn the roll tops over the cheese, cut side down.
Melt the butter in a small bowl. Stir in the parsley and garlic powder until combined. Use a pastry or basting brush to spread the butter mixture all over the tops of the sliders. (You may have some extra butter.)
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 immediately.
If you have leftover holiday ham or turkey, you can use that too! Just slice it thin enough that the sliders will be easy to bite into.Other delicious topping ideas: