A Potato gratin is not the same as scalloped potatoes. A gratin relies on the potatoes alone to provide the starch for the dish and is usually only baked in the oven and not precooked.
Potato gratin is an old classic French dish. The original dish is the simplest version of all: potatoes, cream and a hint of garlic. Cheese is not included although it has often been added in newer versions. Personally, I usually don’t use cheese for a few reasons: First and foremost, I like simplicity in food. Secondly, I use mostly cream for the sauce in this dish, which results in rich creaminess – cheese is not needed. If you do want to add cheese, don’t use a mild cheese, the flavour would not come through and will not be French. Use Gruyère, which is the most common cheese used for cooking in France and will complement the creaminess of the dish with its nutty flavour.
As you can see, the cream will be nicely browned on top even without the use of cheese. In the original, the dish is buttered and a clove of garlic is cut and rubbed all around the dish to obtain just a hint of flavour. I prefer to crush the garlic into the sauce since it is the main flavouring.
Over time, I have changed my approach to this dish. I used to mix 75% milk and 25% cream for the sauce to cut down on the fat, whereas now, I use mostly cream. If I want to lighten the dish up a bit, I mix a little chicken broth into the cream (no more than 20%) but not enough to take away from the simplicity of the ingredients. The problem with adding milk is that it often curdles. Potatoes have actually a lot of vitamin C, and the acid curdles the milk when heated too high, much like a custard. To avoid curdling, potato gratin should be cooked in a slow oven.
Here, I used a 29 cm diameter and 6 cm high pie dish. The amount of liquid will vary slightly depending on the size of your dish, so this recipe is more of a guideline. In this picture you see how much of the potatoes should be covered by cream before baking.
3 lbs potatoes
2 cups cream or more, depending on your dish
2 medium cloves of garlic
salt and pepper to taste
a dash of nutmeg
Preheat the oven to 300 degrees.
Peel and slice the potatoes thinly and arrange in the dish. Sprinkle with salt.
Crush the garlic into the cream, season with salt, pepper and nutmeg, pour over the potatoes and bake for 1 1/2 to 2 hours, depending on your oven and the depth of your dish. Test with a fork the softness of the potatoes.
After baking, let rest for about 10 minutes before serving. This helps to absorb all the liquid.