Preheat the oven on 375º. Carefully slice the sliders in half as evenly as possible. Place some parchment on a rimmed baking sheet.
Lay the bottom "sheet" of the sliders on the parchment paper. In a medium bowl, add the mayonnaise, dijon, horseradish. Mix well, give it a taste. Add more mayo or more horseradish as needed.
Brush the sliders. Brush half the sauce on the bottom of the sliders and crush the other half on the tops of the sliders.
Build the sliders! Start with the roast beef, feel free to chop it up for easier serving, but this is optional. Add onions, provolone slices.
Add the sheet of "tops" of the sliders. In a medium bowl, add melted butter, seasonings, Worcestershire sauce and horseradish (if using some extra). Brush the tops of the sliders well.
Bake for 8-10 minutes, or until tops are golden and cheese is melted.
Remove from the oven, then let sit for 5 minutes. Transfer to a cutting board, then use a serrated knife to cut sliders apart.