Preheat the oven to 350°F and generously grease a 9×13-inch pan with cooking spray, then line it with parchment paper. Using a serrated knife, slice the slab of Hawaiian rolls in half horizontally, keeping all the rolls connected.

In a small bowl, whisk together the mayonnaise and horsey sauce, if using, then spread the mixture evenly over the cut sides of both the top and bottom halves of the rolls.

Place the bottom half of the rolls in the prepared pan and layer with 6 slices of Havarti cheese, followed by the roast beef, then top with the remaining 6 slices of cheese.

Bake uncovered for 10 minutes, just until the cheese begins to melt.

While the sliders bake, whisk together the melted butter, dried minced onion, Dijon mustard, Worcestershire sauce, poppy seeds, garlic powder, and kosher salt until fully combined.

Remove the pan from the oven and place the top half of the rolls over the melted cheese. Pour the herb-butter mixture generously over the tops and sides of the rolls, brushing to coat the bread evenly and allowing some butter to pool in the pan if needed.

Cover the pan loosely with foil and bake for 15 minutes, then remove the foil and continue baking for 5 to 7 minutes, or until the tops are golden brown and the cheese is fully melted.

Remove from the oven, cut into individual sliders, sprinkle with chopped parsley, and serve hot with pickles, fries, or a side salad.