Preheat oven to 375 degrees. Spray a 9x13 baking dish with non-stick spray.
Add the butter to a large sauce pan over medium heat and melt. Stir in the onions and cook until beginning to soften, about 5 minutes.
Sprinkle the flour over the onions and cook, stirring constantly, for 1 minute.
Pour in the milk and cream while whisking the mixture constantly. Continue cooking over medium heat, whisking constantly, until mixture begins to thicken, about 3 minutes.
Remove from the heat and stir in the grated cheddar. Stir until cheese has melted and mixture is smooth.
Stir in the cream of chicken soup, sour cream, salt, parsley, and pepper until combined.
Place half of the potatoes in a thick layer in the prepared 9x13. Pour half of the cheese sauce over the top. Repeat the layers.
Cover tightly with foil and bake for 30 minutes.
Remove the foil carefully and continue baking for 45 minutes or until potatoes are soft.