01 - Preheat oven to 350°F. Line a baking sheet with parchment paper.
02 - Slice slider buns in half horizontally. Arrange bottom halves on prepared baking sheet.
03 - Spread mustard evenly over bottom halves of buns. Add mayonnaise if using.
04 - Layer sliced corned beef over mustard. Top with Swiss cheese and shredded cabbage or sauerkraut if desired.
05 - Place top halves of buns over fillings to complete sliders.
06 - Mix melted butter with poppy seeds, garlic powder, and onion powder in small bowl.
07 - Brush tops of buns generously with butter mixture.
08 - Cover sliders with foil and bake for 7 minutes.
09 - Remove foil and bake for additional 3 minutes until tops are golden and cheese is melted.
10 - Remove from oven, let cool slightly, and serve warm.