01 - Combine chopped corned beef, drained sauerkraut, shredded Swiss cheese, softened cream cheese, Dijon mustard, and black pepper in a large bowl. Mix thoroughly until all ingredients are evenly distributed.
02 - Shape the mixture into 1-inch balls, approximately 24 total. Arrange on a parchment-lined baking sheet and freeze for 20 minutes to firm the texture for easier handling.
03 - Prepare three shallow bowls: place flour in the first, beat eggs in the second, and pour panko breadcrumbs into the third. Line them up in order for efficient coating.
04 - Roll each chilled ball in flour to coat evenly, dip into beaten egg, then press firmly into panko breadcrumbs to ensure complete coverage. Return to the prepared tray.
05 - Pour vegetable oil to a depth of 2 inches in a deep skillet or saucepan. Heat to 350°F, maintaining temperature throughout frying for optimal crispness.
06 - Fry breaded balls in batches of 6-8 for 2-3 minutes, turning occasionally, until deep golden brown and crispy. Remove with a slotted spoon and drain on paper towels.
07 - Whisk mayonnaise, ketchup, dill pickle relish, horseradish, hot sauce, smoked paprika, garlic powder, salt, and pepper in a small bowl until completely smooth and well combined.
08 - Arrange warm Reuben balls on a serving platter alongside the spicy Thousand Island sauce. Serve immediately while crispy for best texture.