01 - Combine chopped corned beef, sauerkraut, Swiss cheese, cream cheese, Dijon mustard, and black pepper in a large bowl. Mix until evenly blended.
02 - Roll the mixture into 1-inch balls and place on a parchment-lined tray. Freeze for 20 minutes to firm up for easier handling.
03 - Arrange three shallow bowls: place flour in the first, beat eggs in the second, and panko breadcrumbs in the third.
04 - Coat each ball in flour, dip in beaten egg, then press into panko breadcrumbs to coat thoroughly.
05 - Pour 2 inches of vegetable oil into a 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 with spicy Thousand Island sauce for dipping.