Preheat the oven to 350°F (175°C).

Grease a baking dish with cooking spray or butter.

Slice the slider buns in half, keeping tops and bottoms connected.

Layer a slice of ham on each bottom half of the buns, followed by a slice of Swiss cheese.

Place the top halves of the buns back on.

In a mixing bowl, combine the melted butter, Dijon mustard, poppy seeds, and minced onion.

Brush the butter mixture generously over the top of the assembled sliders.

Cover the baking dish with aluminum foil and bake for 20 minutes.

Remove the foil and bake for an additional 5 minutes.