Preheat the oven to 350 F and set the bottom half of the slider rolls into a 9x13 inch baking dish, keeping them connected.

Stir the mayonnaise and horseradish together and spread it evenly over the cut side of the bottom buns.

Layer the roast beef evenly over the buns, then top with the cheese slices.

Set the top half of the rolls back on, keeping the sheet connected.

Whisk the melted butter, garlic powder and parsley together and brush it generously over the tops of the rolls.

Cover the dish with foil and bake for 10 minutes, then uncover and bake 2 to 3 minutes more, until the cheese is fully melted and the tops are golden.

Let the sliders rest for 5 minutes, then cut along the roll seams into individual sandwiches before serving.