In a large pot, add chicken stock - bring to a boil over med-high heat.
Whisk together milk and flour until smooth.
Pour the milk mixture slowly into the boiling chicken stock, whisking to combine.
Whisk in salt, pepper, garlic powder, dry minced onion,and thyme.
Reduce heat to medium bringing the mixture back to a boil, stirring constantly so that the bottom of the pan does not burn.
Let boil slowly for about 3 minutes or until thickened. At this point you can stir in the chicken. Remove pan from the heat. It will be VERY thick.
If you're not using the condensed soup in a recipe immediately, transfer it to airtight containers and store in the refrigerator for up to 1 week. It does not freeze well.
Keep in mind, that this recipe is NOT meant to be consumed as it is. It is a concentrated soup, which means that you need to dilute it to be eaten as a soup, or mixed into a recipe that calls for condensed soup.