Step 1: Prep and Preheat
Preheat your oven to 350°F. Spray a 9×13-inch baking dish with non-stick cooking spray or line with parchment paper. Drain the sauerkraut in a colander, then place it on several layers of paper towels and squeeze out as much liquid as possible—this step is critical for preventing soggy sliders.

Step 2: Slice the Rolls
Without separating the rolls, use a long serrated knife to slice the entire block of connected rolls horizontally, creating a top half and bottom half. Keep them connected—this makes assembly so much easier! Place the bottom half in your prepared baking dish.

Step 3: Layer the Filling
Spread half of the Thousand Island dressing evenly over the bottom layer of rolls. Layer the corned beef evenly over the dressing, covering all the rolls. Distribute the well-drained sauerkraut evenly over the corned beef. Layer the Swiss cheese slices over the sauerkraut, overlapping as needed to cover everything. Spread the remaining Thousand Island dressing on the cut side of the top roll half, then carefully place it on top of the cheese layer.

Step 4: Make the Butter Topping
In a small bowl, combine the melted butter, Dijon mustard, garlic powder, onion powder, and a pinch of salt. Whisk together until well combined.

Step 5: Brush and Season
Use a pastry brush to generously brush the butter mixture all over the tops of the rolls, making sure to get into all the crevices. Sprinkle poppy seeds or sesame seeds over the top if using.

Step 6: Bake Covered
Cover the baking dish tightly with aluminum foil. Bake for 15 minutes—this allows the cheese to melt and the flavors to meld together without the tops browning too quickly.

Step 7: Bake Uncovered
Remove the foil and continue baking for 5-7 minutes more, or until the tops are golden brown and slightly crispy. The cheese should be fully melted and bubbly around the edges.

Step 8: Rest and Serve
Remove from the oven and let the sliders rest for 2-3 minutes. This allows everything to set slightly, making them easier to cut and serve. Use a sharp knife to cut along the pre-scored lines between each roll to separate into individual sliders. Serve hot with extra Thousand Island dressing on the side for dipping.