In a medium saucepan melt butter over a medium-low heat.
Add the flour and whisk together until well combined. Cook for 1-2 minutes to cook out raw flour taste.
Add chicken stock and milk. Whisk until smooth, about 1-2 minutes.
Season with salt, pepper, garlic powder and onion powder.
Bring to a simmer and cook until it begins to thicken, about 3-5 minutes.
Add in the chopped celery and stir to combine.
Remove from heat and allow to cool completely.
Store in airtight container in refrigerator.
Use as you would a can of condensed soup.