In a small bowl, mix the melted butter, Dijon mustard, garlic powder, and a pinch of salt and pepper.
Slice the slider buns in half horizontally without separating them. Place the bottom halves in a greased baking dish.
Layer half of the sliced ham evenly over the bottom buns. Top with a layer of Havarti cheese slices.
Add the remaining sliced ham on top of the cheese, then cover with the top halves of the buns.
Pour the mustard butter mixture evenly over the top of the sliders, ensuring they are well-coated. Sprinkle poppy seeds on top for added flavor and crunch.
Cover the baking dish with aluminum foil and bake in the preheated oven for 15 minutes.
Remove the foil and bake for an additional 5-10 minutes, or until the cheese is bubbly and the tops of the sliders are golden brown.
Remove from the oven and let cool for a few minutes. Carefully lift the sliders from the dish using a spatula.
Garnish with fresh arugula or spinach, then slice them to serve.