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 mixture into 1-inch balls and place on a parchment-lined tray. Freeze for 20 minutes to firm up.
03 - Arrange three shallow bowls: flour in one, beaten eggs in second, panko breadcrumbs in third.
04 - Coat each ball in flour, dip in egg, then coat thoroughly with panko breadcrumbs.
05 - Heat 2 inches of vegetable oil in a deep skillet or saucepan 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.