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