Using a food ricer, rice the potatoes into a large mixing bowl. Add the sauerkraut, ham, egg yolks, 1/4 cup of mustard and chopped green onions. Mix well. Season with salt and pepper. Form the mixture into small balls, about a tablespoon each. Place on a parchment lined baking sheet. Place in the freezer and freeze for 30 minutes.
Preheat the fryer. Season the flour, egg wash, and bread crumbs, with salt and pepper. Roll each sauerkraut ball in the flour, coating completely. Dip each in the egg wash, letting the excess drip off. Finally roll each in the bread crumbs. Fry the sauerkraut balls in batches until golden brown, about 2 to 3 minutes. Remove from the fryer and drain on paper towels. Season with salt.
In small mixing bowl, combine the remaining 1/2 cup of the mustard and sour cream. Season with salt and pepper. Mix well. Serve the sauerkraut balls on a large platter with the mustard dipping sauce. Garnish with parsley.