Place the mashed potatoes in a mixing bowl. Add the sauerkraut, egg yolk, mustard, and chopped green onion. Mix well and season to taste with the salt and pepper.
Form the balls using the small scoop (approximately 2 Tbsp). Place on a parchment-lined sheet pan and chill for at least 30 minutes.
Set up a breading station by seasoning the flour with seasoned salt in a shallow bowl.
In a separate bowl, whisk the eggs with milk.
In a shallow bowl or plate, fill with the bread crumbs.
Roll each cold ball into the flour, then the egg wash, and roll in the breadcrumbs.
Fry until golden brown and delicious, about 2 -3 minutes. Serve with sauce if desired.