Preheat oven to 350 degrees F.
Prepare the slider rolls: Slice slider rolls in half horizontally, keeping them connected if possible. Arrange the bottom halves in a single layer on a large rimmed baking sheet (about 13x18 inches).
Make the butter sauce: in a small bowl whisk together melted butter, poppy seeds, finely diced onion, dijon mustard, and Worcestershire sauce until well combined.
Assemble the sliders: evenly layer the sliced ham over the bottom halves of the rolls. Place the Swiss cheese on top of the ham. Use a spoon to spread the butter sauce evenly on the underside of the top halves of the rolls, then place the tops onto the sliders, buttered-side down.
Cover and bake: cover the tray with foil and bake for 10-15 minutes, or until the cheese is melted and the sliders are warmed through.
Remove from the oven, separate the sliders and serve.
