In a small bowl, whisk together flour, onion powder, garlic powder, garlic salt and pepper.
Add milk and whisk until combined, set aside.
In a 4-quart pan over medium heat, boil water.
Add chicken bouillon and whisk until dissolved.
Add milk/dry ingredient mixture to boiling water/bouillon.
Cook over medium heat, stirring every 30 seconds or so, until thick (about 3 minutes).
Once the sauce is thick, remove from heat.
Add diced chicken breasts, cubed potatoes and frozen veggies to the pot of your slow cooker.
Pour soup mixture into crock pot and stir to combine.
Cook on high for 6 hour, stirring occasionally.
Towards the end of cooking, if you notice your mixture isn’t thickening, turn the lid so there’s a little crack where steam can escape. Letting the moisture out as it finishes cooking will thicken it up!