Melt butter in large saucepan over medium heat. Add onion; cook, stirring occasionally, for 1 to 2 minutes or until tender. Stir in flour. Gradually stir in broth and evaporated milk. Scoop potato pulp from 1 potato (reserve potato skin); mash. Add pulp to broth mixture. Cook over medium heat, stirring occasionally, until mixture comes just to a boil. Dice remaining potato skin and potato(es); add to soup. Heat through. Season with salt and ground black pepper. Top each serving with bacon, cheese and green onions, if desired.