Add the mayonnaise, ketchup, pickle relish, and lemon juice to a small bowl and whisk to combine.
To assemble the sliders:
Preheat the oven to 350 degrees Fahrenheit. Slice the top half Hawaiian rolls off and set aside, place bottom half on a baking sheet.
Spread the dressing evenly onto the bottom half of the Hawaiian rolls.
Layer on the corned beef and Swiss cheese. Squeeze the sauerkraut dry (you can squeeze dry in your fist or with a paper towel) and distribute on top. Place the top half of the rolls on top.
To bake:
In a small bowl, combine the melted butter, poppyseeds, and minced onion. Use a pastry brush to brush the butter mixture generously on top of the rolls. Cover with foil and bake for 15 minutes.
Remove foil and bake for an additional 8-10 minutes, or until the cheese is melted and the tops are lightly toasted. Remove from the oven and carefully cut the sliders. Serve warm.
Notes
I used Kings Hawaiian brand for this recipe. You can use regular rolls as well if you prefer to not have the sweetness of Hawaiian rolls.If you love the flavor of rye bread, try adding a pinch of two of ground caraway to the butter mixture. It'll add a bit of that distinct rye flavor.You can use bottled Thousand Island dressing instead of making your own if you prefer.