Preheat oven to 375. Lightly grease/spray a 13" x 9" baking pan.
Place thinly sliced potatoes in a large mixing bowl. Add onion, flour, salt, white pepper, paprika, ground mustard, parsley and 1 1/2 cups of the shredded cheddar. Toss to mix well. Place this mixture into the prepared baking dish.
Pour heavy cream and milk over top. Sprinkle with the remaining 1 cup of shredded cheddar cheese.
Cover and bake at 375 for 50-60 minutes. Uncover and bake an additional 25-30 minutes or until top is to desired browness/bubbliness.