To prepare, place margarine in a Dutch oven over medium heat until melted. Add potato, onion, and celery to Dutch oven; cook, stirring frequently, 5 minutes. Add broth and next 4 ingredients; bring to a boil. Cover, reduce heat and simmer 15 minutes.
Remove 2 cups vegetables from broth with a slotted spoon, partially mash vegetables with a fork or potato masher. Return vegetables to Dutch oven. Add turkey and milk; bring to a boil over medium heat, stirring constantly.
Combine flour and water, stir until smooth. Slowly add flour mixture to Dutch oven, cook over medium heat, stirring constantly 4-5 minutes or until thickened and bubbly.