Preheat a large slow cooker by turning it to the High setting and covering with the lid.
Place the cut-up potatoes in a microwave safe bowl and microwave on High for about 8 minutes until the potatoes are cooked and steaming hot.
While the potatoes are cooking place the butter in the preheated slow cooker and cook and stir the onions and garlic for about 5 minutes or until the onions turn golden.
Stir in milk and whisk in the flour until smooth. Gradually whisk in the remaining 3 cups of milk and let the mixture come to a simmer in the slow cooker. Simmer for about 10 minutes until the soup begins to thicken.
Stir in the hot cooked potatoes crumbled bacon, sour cream, cheddar cheese, parmesan cheese, seasoned salt, parsley, chilli flakes, celery salt and dried basil.
Stir to mix the soup well then reduce the slow cooker setting to Low then cover and cook for 3 hours stirring occasionally. Sprinkle with chives and serve.