Bring a large saucepan of salted water to boil and place potatoes in the boiling water. Let potatoes cook until they are just becoming tender; approximately 8 to 10 minutes. Refrigerate the potatoes overnight.
Preheat oven to 180 degrees C and grease a baking dish.
Grate the potatoes. In a small bowl combine 1 teaspoon parsley, 1 teaspoon salt, 1/4 teaspoon pepper, 1 tablespoon minced onion and 1/2 of the butter. In a separate small bowl combine sour cream and milk.
Layer 1/2 of the grated potatoes into the bottom of the prepared baking dish. Sprinkle the spice mixture over the layer of potatoes. Pour 1/2 of the sour cream and milk mixture over the potatoes and butter layers. Top this layer with 1/2 cup of cheese. Repeat the layering process one more time.