The recipe for boulangere potatoes emerged from the French tradition of taking a casserole of potatoes to the local baker at the end of the day so the casserole could be placed in the baker's cooling oven to cook overnight. The dish involves baking thinly sliced potatoes that have been layered in a pan with onions and then bathed in stock. After an hour or more of cooking, the top layer of potatoes has browned and become crisp, while the lower layers have absorbed the stock and become thick and moist. Boulangere potatoes are often served as an accompaniment to roasted meats and can make for a dramatic presentation when the potato slices are arranged in complex patterns inside the baking dish.
The cooking method for making boulangere potatoes works best when the potatoes have been cut into very thin chips. This is most often achieved through the use of a mechanical cutting device like a mandolin or a food processor. The type of potatoes used is not necessarily important to the end result, as long as the flavor of the potatoes is acceptable to the chef.
After cutting the potatoes into thin slices, onions are cut into equally thin slices, although it is not important to keep the rings of the onions intact. Many recipes for boulangere potatoes call for the onions to be sauteed in olive oil and butter until they have turned translucent but have not fully caramelized. These same recipes sometimes fry the potatoes in the pan until they have browned slightly. This extra step can allow the potatoes to retain some texture as they cook in the oven, and it also can reduce the overall cooking time. The onions are cooked in advance because, when layered between the potatoes, they do not always have enough time to fully cook.
In an oven-safe baking dish, a layer of potatoes is placed on the bottom of the dish. This is followed by a layer of onions; herbs such as parsley, thyme or rosemary; and then potatoes again. The sequence is repeated until all the potatoes and onions are used, with the top layer being formed from potatoes. Chicken, veal or vegetable stock is warmed in a pan with salt, pepper and garlic and then strained into the cooking dish until the to of the liquid meets the top of the potatoes but does not fully cover them. The boulangere potatoes are then placed into a hot oven and baked, covered, for an hour, after which the cover is removed and the dish continues to bake until a crust has formed on the top.
When the boulangere potatoes have finished cooking, the stock will have been absorbed by the potatoes and the top layer will be crisp. The crispness is sometimes encouraged during baking by the addition of butter. The dish is often seen served with roasted meats such as lamb, veal or beef.