Preheat oven to 350 degrees.
Coat a 9 X 13 inch baking dish with non-stick spray.
Layer the potato slices in the dish and sprinkle the chopped onion on top of the potatoes.
In a medium bowl, mix together the sour cream, milk, cheese, garlic, salt, and pepper.
Pour the mixture over the potatoes and onions.
Sprinkle paprika on top.
Cover with foil and bake for 30 minutes.
Remove foil and bake for another 30 minutes.