01 - Preheat the oven to 350°F. Line a baking sheet with parchment paper.
02 - Slice the slider buns in half horizontally. Place the bottom halves on the prepared baking sheet.
03 - In a small bowl, mix Dijon mustard and mayonnaise until combined. Spread the mixture evenly over the cut sides of the bottom buns.
04 - Layer each bottom bun with corned beef, a slice of Swiss cheese, a pickle slice, and a few red onion rings if using.
05 - Place the top halves of the buns over the fillings.
06 - In another small bowl, combine the melted butter with garlic powder, poppy seeds, and onion flakes.
07 - Brush the tops of the buns generously with the butter mixture.
08 - Cover the sliders loosely with aluminum foil and bake for 8 minutes.
09 - Uncover and bake for an additional 2-3 minutes, until the cheese is melted and the tops are golden. Serve warm.