Preheat waffle maker to the highest heat.
Liberally salt potatoes and lay them out on a stack of paper towels to drain. Stack a few paper towels on top, press down and let sit for a few minutes.
While potatoes drain, whisk together the onion, egg, sour cream, horseradish and pepper in a large bowl. Add drained potatoes to bowl and fold until combined.
Brush hot waffle maker with melted butter. Evenly spread potato mixture around the waffle maker. Close and cook for 12 minutes or until potatoes are crispy and brown.