Two color decadent scalloped potatoes

I came across a recipe in one of my magazines the other day for a sweet potato and white potato scalloped potato. It sounded like such a good idea that I decided to make it for Christmas dinner since I was making ham. When it came time to make the dish, I couldn't find the recipe. Most of the ones I found online used cheese and in my opinion, that is au gratin not scalloped. So I decided to just modify a regular scalloped potato recipe.

2 sweet potatoes peeled
5 white potatoes (or an amount to equal the sweet potatoes) peeled
1 large sweet onion
3 tablespoons flour
1 tsp salt 
1/4 tsp pepper
4 tablespoons butter
1/4 teaspoon nutmeg (I ground mine fresh)
21/2  cups milk
1/2 cup heavy cream

I use a mandolin to slice my potatoes and I sliced them the day before and stored them in water. Pat dry. Slice the onion, I also used the mandolin. Combine flour, salt, pepper, and nutmeg.

Layer 1/4 of the potatoes and onions in the bottom of a greased dish.sprinkle 1 tablespoon of flour, salt, pepper, and nutmeg mixture. place 1/4 of butter in pieces. 

Repeat till all in the ingredients are in the pan. Top with the last of the butter, 

Warm the milk to scalding and pour over. I used 1% milk and this did not give the rich results I wanted. I corrected that by adding 1/2 heavy cream after 1 hour 45 minutes and cooking for another 30 minutes. 

This took a lot longer to cook than I expected. I cooked it at 375 for 30 minutes covered and then at least an hour and a half uncovered. I like a crispy top. Adding the heavy cream made it just perfect, rich, creamy and delicious. 

Everyone loved this dish and I would definitely make it again.