01 - Combine chopped corned beef, sauerkraut, Swiss cheese, cream cheese, Dijon mustard, and black pepper in a large bowl. Mix until thoroughly blended.
02 - Shape mixture into 1-inch balls and arrange on parchment-lined tray. Freeze for 20 minutes until firm.
03 - Arrange three shallow bowls: place flour in first, beaten eggs in second, panko breadcrumbs in third.
04 - Coat each ball in flour, dip into egg mixture, then press firmly into panko crumbs to coat completely.
05 - Pour 2 inches vegetable oil into deep skillet or saucepan. Heat to 350°F.
06 - Fry balls in batches, turning occasionally, until golden brown and crispy, about 2–3 minutes per batch. Drain on paper towels.
07 - Whisk together mayonnaise, ketchup, pickle relish, horseradish, hot sauce, smoked paprika, garlic powder, salt, and pepper until smooth.
08 - Serve Reuben balls warm accompanied by spicy Thousand Island sauce for dipping.