Combine 1/2 cup of the cold water with flour in a medium bowl and whisk until well blended. Set aside.
Pour remaining water and milk into a large pot and slowly bring to a boil. Add celery, onion, mushrooms, chicken bullion, thyme, fresh pepper, frozen vegetables and return to a boil. Partially cover and simmer on low until vegetables are soft, about 20 minutes. Remove lid, add potatoes and cook until soft, about 5 minutes. Add chicken, and slowly whisk in slurry, stirring well as you add. Cook another 2-3 minutes, until soup thickens, adjust salt and pepper to taste and serve.
from Skinny Taste-recipe and picture