Arrange a rack in the middle of the oven and heat the oven to 350°F.

Place 4 tablespoons unsalted butter in a small, microwave-safe bowl and microwave until melted, 20 to 30 seconds. Add 1 tablespoon Dijon mustard, 1 teaspoon Worcestershire sauce, ½ teaspoon onion powder, and ¼ teaspoon kosher salt, and whisk to combine.

Without separating the rolls, cut 1 package Hawaiian sweet dinner rolls in half horizontally with a serrated knife. Place the bottom half of the rolls in a 9x13-inch or 7x11-inch baking dish.

Arrange 6 slices of the Swiss cheese on the rolls, overlapping the slices as needed to completely cover. Fold and arrange 12 thin slices ham over the cheese, arranging a slice on each roll. Layer the remaining 6 slices Swiss cheese over the ham. Place the top half of the rolls over the cheese.

Brush the butter onto the top of the rolls, then sprinkle with 1 teaspoon poppy seeds. Cover the baking dish tightly with aluminum foil.

Bake until the sandwich is heated through and the cheese melts, about 20 minutes. Uncover and bake until the rolls are lightly browned, 7 to 8 minutes. Transfer the slab to a cutting board, then cut into individual sliders with a serrated knife before serving.