My wife made this potato gratin dish (I helped with cutting potatoes and making the Béchamel sauce) as a side when we made a roasted pork tenderloin with mustard, ginger, and garlic marinade. The original recipe for the potato dish is from “Cooking light” but I do not think this is quite “light” cooking. I just used the leftover potato gratin as a part of Izakaya course menu item.
For the potato gratin, peel and slice both potatoes and sweet potatoes (2 medium, each) in 1/4 inch in thickness. Boil 4-5 minutes in salted water until just cooked. For Béchamel sauce, saute one strip of bacon finely cut until the fat is rendered and the bacon gets crispy. Take out the bacon and set aside. Add 1 tbs of butter and saute chopped onion (one medium) for 1-2 minutes (do not brown) and add 1/3 cup of all purpose flour. Keep sauteing so that all the onion pieces are coated with flour (using finely chopped onion is the secret of making Béchamel with a small amount of fat since each piece of onion holds flour on its surface and prevents the flour from clumping). Add 2 cups of low-fat (I used 1%) milk at once and stir on medium low heat. Add back the bacon. As the liquid heats up, it thickens. Add more milk if it is too thick. Stir in 1/2 cup of grated Parmigiano-Reggiano cheese and adjust the seasoning (salt and pepper). The final product should be rather loose (if needed, add more milk). Grease a rectangular (9×13 inch) pyrex baking dish with light olive oil. Make alterate layers of the cooked potatoes and sweet potatoes slices and pour the Béchamel cheese (Mornay) sauce over the potatoes to cover. Bake at 370F for 45 minutes.
For this serving, cut small squares of the leftover potato gratin and put it in toaster oven at 400F for 5 minutes or until throughly heated. Saute green asparagus (pre boiled) in browned melted butter and season (salt and pepper). We used this as a part of the Izakaya course for our guests. Although this is not Japanese dish it went very well with the rest of the Japanese dishes we served. Such a comfort food.