Lightly grease the container of a slow cooker using cooking spray or a little oil.
Whisk together the whole milk, all-purpose flour, onion powder, garlic powder, salt, and pepper in a small bowl.
Bring the water to a boil in a saucepan over medium heat and add the chicken bouillon paste. Whisk until fully dissolved.
Add the milk and flour mixture to the boiling water and whisk until smooth. Continue to cook over medium heat for about 3 minutes, or until it thickens.
Remove the saucepan from heat and add the cubed chicken, red potatoes, and frozen mixed vegetables to the prepared slow cooker.
Pour the thickened soup mixture into the slow cooker and stir everything together.
Cover the slow cooker and cook on high for 3-4 hours or low for 6 hours, stirring occasionally.
During the last 30 minutes of cooking, slightly ajar the lid to allow the soup to thicken.
Serve the soup warm, garnished with additional black pepper or parsley if desired.