Preheat the oven to 350°F and lightly grease a baking dish.
Slice the slider buns in half and place the bottom halves into the prepared baking dish.
Spread the mayonnaise evenly over the bottom buns.
Layer half of the Swiss cheese, followed by the ham, then the remaining Swiss cheese. Place the top halves of the buns over the filling.
In a small bowl, mix the melted butter, Dijon mustard, minced onion, and poppy seeds until well combined.
Evenly brush the butter mixture over the tops of the sliders, ensuring full coverage.
Cover the dish with foil and bake for 15 minutes. Remove the foil and continue baking for another 5–10 minutes, or until the tops are golden brown.
Let cool slightly before serving.