Create a slurry by combining 0.5 cups of 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 bouillons, 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 to 3 minutes, until soup thickens, adjust salt and pepper to taste and serve.