01 - Preheat oven to 350°F. Line a baking sheet with parchment paper.
02 - Slice slider buns in half. Place bottom halves on the prepared baking sheet.
03 - Layer each bun bottom with corned beef, a halved slice of Swiss cheese, a few onion rings, and a pickle slice.
04 - Spread whole-grain mustard (and mayonnaise, if using) on the inside of each top bun.
05 - Place the top buns over the fillings.
06 - Mix melted butter, Dijon mustard, garlic powder, and poppy seeds (if using) in a small bowl. Brush generously over the tops of the buns.
07 - Cover sliders loosely with foil and bake for 10 minutes. Remove foil and bake for 5 more minutes, until the cheese melts and tops are golden.
08 - Serve warm.