Peel potatoes and cut into ½-inch dice. Cook in 2½cups water with the bay leaf, pepper and salt till potatoes are half done.
Heat the lard. Brown the flour and onion in lard till medium golden in color. Add ½cup cold water and stir till smooth. Pour into the pot in which potatoes are cooking and cook over very low heat till potatoes are done. Stir every now and then.
In a bowl mix well the sour cream, sugar, parsley, and vinegar to taste. Pour into the potatoes, bring to a boil, and serve.