In a saucepan, combine potatoes, onion, water, celery, and bouillon granules. Bring to boiling; reduce heat. Cover; simmer 20 minutes or until potatoes are tender. Mash potatoes slightly (don't drain).
In a medium mixing bowl, combine dry milk, flour, and pepper. Gradually add milk, stirring until smooth. Stir into potato mixture. Cook and stir until bubbly. Cook and stir for 1 minute more.
Ladle into soup bowls. Garnish with parsley. Makes 5 side-dish servings.