Preheat oven to 350°F (175°C).
Slice the Hawaiian rolls horizontally in half without separating individual rolls.
Layer the bottom half with slices of black forest ham evenly.
Place sliced Havarti cheese over the ham.
Cover with the top half of the rolls.
In a small saucepan, melt the butter and stir in Dijon mustard, Worcestershire sauce (if using), onion powder (if using), and poppy seeds to create the glaze.
Brush the glaze generously over the top of the rolls.
Place the sliders in a 9x12 baking pan and cover loosely with foil.
Bake for 20 minutes, removing foil halfway through baking for the tops to brown.
If tops brown too quickly before cheese melts, re-cover with foil and continue baking until cheese is melted and sliders are heated through.
Remove from oven, cut sliders apart, and serve warm.