added note: after posting this, I have since learned of a dish called pommes anna that I had never heard of or eaten (but I guess re-created with a twist). This dish is much more of a pommes anna than a gratin, but it is still amazing.
Lightly sweet, perfectly browned, with a hint of caramelized onion, this gratin makes an impressive looking and wonderful tasting accompaniment to any autumn or winter meal. It is fairly easy to prepare, but beware! This one takes some time to bake.
2 Sweet potatoes, scrubbed
3-5 Russet potatoes, scrubbed
1 medium onion
1/3 cup cream
3 T butter
Salt
Onion Powder
Slice your onions in thin 1/2 or 1/4 slices. I always keep the skins attached so that I can use it as a handle when I get down toward the root end. Caramelize your onion in 1 T of butter and a sprinkle of salt.
Slice your potatoes in thin round slices, putting the russet slices in a bowl of hot water. Don't worry if your slices aren't perfect; you will use the best ones for the top and then the rest will not really be visible.
Line a large cast iron skillet with parchment paper. Turn on the pan and melt 1 T of butter on the parchment paper. Turn off the pan. The parchment paper is a key part of flipping the gratin when it is finished.
Layer russet potatoes in the bottom of the pan, arranging them in a circular pattern. This will be the top of your gratin, so use your prettiest slices for this layer. Lightly sprinkle with salt and onion powder.
Next, layer half of the sweet potatoes and half the onions in the pan as evenly as possible.
Finish your layers with russets, salt and onion powder, followed by another later of sweet potatoes and finished finally with a final layer of russets. Drizzle the cream over the top and distribute the last T of butter over the top. Put a round of parchment over the top and bake with another oven safe pan on the top as a weight.
Bake for 1 hour and 15 minutes at 400 degrees. Allow to cool for 15 minutes. Run a knife or spatula around the edges to loosen, and flip onto a platter to serve.
I have found that a serrated bread knife works best to cut this dish into perfect wedges.
Enjoy!