In a large mixing bowl, combine diced corned beef, shredded Swiss cheese, sauerkraut, and Italian seasoning. Mix thoroughly until well incorporated. Form into small balls, about an inch in diameter, and set aside.

Set up a dredging station with three shallow bowls: flour in the first, whisked egg in the second, and breadcrumbs in the third. Heat frying oil in a deep skillet to 350°F.

Roll each ball in the flour, shake off excess, dip in the beaten egg, then coat with breadcrumbs, pressing gently to adhere. Repeat until all the balls are coated.

Carefully add the coated Reuben balls to the hot oil in batches, frying for about 3-4 minutes on each side until golden brown and crispy.

Remove fried balls from the oil and place on paper towels to absorb excess oil. Serve hot with zesty Dijon mustard.