HOT TO BAKE POTATOES EN CASSEROLE
Baking potatoes en casserole is the basic thing to do in gratins or in dauphinoise potatoes. Baking en casserole simply means that potatoes are sliced and baked in the oven in a creamy tasty liquid. The baking temperature and timing is set so that the potatoes are cooked very tender but still hold their shape when served. And the best way to get potatoes very tender is to make them absorb as much liquid as possible, therefore, old potatoes with low water content and lots of starch (like Idaho) are most suitable for en casserole dishes. Another type of potatoes that works quite well en casserole is the Yukon Gold or other yellow waxy, and fairly starchy potatoes. Serving potatoes en casserole on their own may not be very exciting, although if you make the sauce very flavorful and have enough variations in it, then the dish can stand on its own. However, most people want something besides them, e.g. meat or fish.
For baking potatoes en casserole you will then need the following: starchy potatoes or waxy potatoes, the flavorful liquid of your choice, and if you also want - some cheese and perhaps some eggs. Basically, you need about half as much liquid (in weight) as you have of the potatoes. Now, the flavorful liquids differ for all recipes and they should be the source of information for your dish. In other words, it won't be covered here, although some general tips can be given if you want to prepare your own dish from scratch. Usually, a mixture of milk and cream is used (proportions depend on your desires, but 3:1 is common). This can be flavored with e.g. garlic, herbs, butter, and well...almost anything that goes well with potatoes. Simply bring the milk and cream to simmer, add the flavoring ingredients, remove from heat and infuse for some time. Then, if you wish, remove the flavoring ingredients, season with salt, pepper, and maybe even some nutmeg, and there you have your en casserole cream! Now, reheat the cream before you bake it with the potatoes, to make the cooking time faster.
When it comes to other flavorings, salt and pepper are standard and should not be forgotten. For cheeses, use the fine ones like Gruyere, Pecorino, or Parmesan - they are nice. You can also mix the potatoes with some other ingredients, like mushrooms or herbs. Once you have created your dish in your mind and prepped it, then it's time to make the casserole!
So, peel the potatoes and slice them, preferably on a mandoline. You can then precook the slices (or whole potatoes, and then slice them) to get the casserole bake faster. This may be necessary if you don't wish to burn your sauce or cheese while waiting for the potatoes to cook through once you put everything in the oven. Now, preheat the oven to 150 C/300 F and grease the baking pan with oil or butter. Layer the potato slices evenly in this pan and add any ingredients if you want to. Once all the layers are completed, pour on the cooking liquid (cream or sauce) and shake the pan slightly to distribute it. Remember that the liquid should be very warm already. Then add any topping ingredients over the casserole (cheese or veggies, etc.) and put it into the oven. The baking time for potatoes en casserole should be adjusted so that the potatoes are tender and the topping is golden brown. If the potatoes cook too quickly, before the topping is ready then just put it briefly under a red hot grill to quickly brown it. If the topping cooks too quickly (browns before the potatoes are tender) then sprinkle it with cold water, and continue to do so until the dish is finished. Serve your potatoes en casserole hot.