I love love scallop potatoes! there is just something about potato cooked in milk & cream and layered with cheese mmmm!
I really don't have a recipe but I mostly do mine this way.
I prefer to pre boil my potato slices (just a bit otherwise mushiness is the result) I layer the potato slices with onions & grated cheese, seasoning with salt & pepper after each layer. I also pour over cream and milk, with the remaining cream & milk poured over the top. its then baked in the oven untill the top is golden brown, delicious :)