Heat butter in a large sauce pan over medium heat. Add onion, celery and cook until translucent, for about 2 minutes.
Add garlic, keep stirring and cook for 1-2 more minutes or until fragrant but do not burn.
Stir in the flour and whisk until lightly browned.
Add chicken stock, milk and stir frequently until the sauce thickens.
Put chicken into slow cooker and pour in the broth/sauce mixture.
Stir in potatoes, poultry seasoning, garlic powder and cook on high heat for 3-4 hours or low for 5-6 hours.
About 30 minutes - 1 hour prior serving, add frozen mixed vegetables and taste the broth. Season with salt/pepper if needed or according to your liking.
Serve with chopped green onions, crispy bacon and rustic bread or biscuits.