01 - Preheat oven to 350°F. Line a baking sheet with parchment paper.
02 - In a small bowl, mix together mayonnaise, Dijon mustard, ketchup, prepared horseradish (if using), onion powder, and black pepper until smooth.
03 - Slice slider buns in half horizontally. Arrange the bottom halves on the prepared baking sheet.
04 - Spread a layer of sauce on each bottom bun. Top each with corned beef, a little sauerkraut (if using), and a slice of Swiss cheese.
05 - Spread a thin layer of sauce on the inside of the top buns, then place them over the cheese.
06 - In another bowl, combine melted butter, poppy seeds, minced onion, Worcestershire sauce, and garlic powder.
07 - Brush the butter mixture generously over the tops of the sliders.
08 - Cover loosely with foil and bake for 10 minutes.
09 - Remove foil and bake an additional 5 minutes, until tops are golden and cheese is melted.
10 - Remove from oven, let cool slightly, then serve warm.