01 - Combine chopped corned beef, sauerkraut, Swiss cheese, cream cheese, Dijon mustard, and black pepper in a large bowl. Mix until evenly blended and sticky.
02 - Roll the mixture into 1-inch balls and arrange on a parchment-lined tray. Freeze for 20 minutes until firm enough to handle without falling apart.
03 - Arrange three shallow bowls: place flour in the first, beat eggs in the second, and fill the third with panko breadcrumbs.
04 - Dredge each chilled ball in flour, shaking off excess. Dip in beaten egg, then press into panko until thoroughly coated.
05 - Pour 2 inches of vegetable oil into a deep skillet or saucepan. Heat to 350°F, maintaining temperature throughout frying.
06 - Fry balls in batches of 6-8, turning occasionally, until golden brown and crispy, about 2-3 minutes per batch. Transfer to paper towels to drain.
07 - Whisk mayonnaise, ketchup, pickle relish, horseradish, hot sauce, smoked paprika, garlic powder, salt, and pepper until smooth and creamy.
08 - Arrange warm Reuben balls on a serving platter alongside spicy Thousand Island sauce for dipping.