Now this may be a little bit of work but most of the real work happens in the oven. While that's going on, you can clean all the pans you used, which I must admit there are quite a few. It is quite colorful looking - rather than that pale looking regular potato gratin.
Two-Potato Gratin (Adapted from Cooking Light)
2 medium baking potato, peeled and sliced 1/4" thick
2 medium sweet potato, peeled and sliced 1/4" thick
2 quarts chicken stock
2 T oil
3 green onions, sliced
2 garlic cloves, crushed
1 T fresh thyme
3 T flour
1 1/2 c milk
1 c chicken stock (reserved from potato liquid)
3 oz Gruyere cheese, grated
1 oz (1/4 c) Parmesan Cheese
Place potatoes in a stockpot with chicken stock. Bring to a boil and cook for 4 minutes.
Meanwhile in a saucepan heat oil and green onions. Saute until softened, add garlic and thyme. Cook until fragrant. Whisk in flour and cook for about 3 minutes. Gradually add milk; whisking constantly.
Remove potatoes from water and spread out on baking sheet. Add 1 c of the chicken stock to the milk mixture and cook until thickened. Remove from heat and add cheese.
Grease a 2 qt casserole dish. Pour a thin layer of the milk into the bottom of the dish. Place a layer of white potatoes on the bottom. Sprinkle with salt and pepper. Next place a layer of sweet potato. Sprinkle with salt and pepper. Shingle the next layer alternating potatoes. Pour milk mixture over the potatoes. Sprinkle with Parmesan cheese.
I left off the Parmesan cheese. It was good without it but in retrospect, it would have added a little more color and taste. This is not a super heavy potato gratin. It does taste rich but it's not artery-clogging rich.