Cut you turkey into small bites, dice up your onion, cut your celery into little pieces. Put all this into a large saucepan. Add your garlic, celery seed, and your butter. Cover with water, not to much just enough to cover it. Bring to a boil and simmer over a medium heat till your onions and celery are tender. Your turkey mixture might cook down some don't worry about it, its ok.
In another saucepan make your white sauce.Melt your butter and stir in the flour, remove from heat. Gradually add your canned milk, stirring continuously with a wire whisk. You want a thick white sauce to add to your turkey mixture.
Slowly add your white sauce to your turkey mixture, bring back to a slow boil. When it gets to the thickness you want remove from heat. Sever over toast or biscuits.