To create the slider spread, whisk the mayonnaise and 2 tbsp Dijon mustard together in a small bowl; set aside.
With a serrated bread knife, halve the slider buns horizontally to detach the tops from the bottoms. Arrange them cut side up on a baking sheet.
Set the oven's broiler to high, placing the rack in the second position from the top (about 6 inches from the broiler element). Broil only the cut sides of the rolls for 1–2 minutes, until lightly toasted. Stay vigilant and do not leave, as they can scorch rapidly; broiling times differ by oven.
Set the oven to 350°F (175°C) with the rack in the middle position. Mist a glass or light-colored ceramic baking dish with cooking spray. If using a darker-interior pan, lower the oven temperature to 325°F (165°C).
Lay the bottom half of the toasted rolls in the prepared baking dish. Apply slightly more than half of the Dijon-mayo mixture onto the rolls in the dish.
Build the sliders by stacking the cheeses and meat in this sequence: cheddar cheese, ham, Swiss cheese. Coat the toasted side of the top half of the rolls with the remaining Dijon-mayo and set them over the Swiss cheese layer.
In a small bowl, mix the melted butter, Worcestershire sauce, poppy seeds, onion powder, and a pinch of salt (if using). Drizzle over the sliders, using a basting brush or pastry brush to spread the butter and poppy seeds uniformly over the tops and sides. It is fine if some butter collects around the sliders in the baking dish.
Cover with foil and let stand for 5 minutes. Bake for 20 minutes, until the sliders are heated through and the cheese is melted. Remove the foil and continue baking for 5–10 minutes, until the tops of the rolls are golden and crispy. Monitor closely during the last stage so they do not get too dark.
Allow the sliders to stand for a few minutes, then cut the rolls into individual servings. Serve hot with potato chips, fries, or your favorite sandwich sides.