01 - Preheat oven to 350°F.
02 - Slice the slider buns in half horizontally and arrange the bottoms on a baking sheet lined with parchment paper.
03 - Layer each bun bottom with corned beef and Swiss cheese.
04 - In a small bowl, mix stone ground mustard with mayonnaise if desired. Spread over the inside of each bun top.
05 - Add a layer of shredded cabbage or sauerkraut and, if using, sliced red onion over the cheese.
06 - Place the bun tops over the fillings. Brush tops with melted butter and sprinkle with poppy seeds if desired.
07 - Bake for 8–10 minutes, or until cheese is melted and buns are golden.
08 - Remove from oven, let cool slightly, and serve warm.