Potato gratin, cream first, cheese last

A gratin is potatoes cooked in cream until the slices are tender and the top is brown. A thick blanket of cheese from the start insulates the potatoes and leaves the middle firm. The cheese goes on for the last 15 minutes.

238 g of heavy cream is 1 cup, loosened with 160 g of milk so the sauce does not split into butter and a dry potato cake. The heavy cream chart is the cup weight. If you only have double cream, use 180 g of it and keep the 160 g of milk. Double cream is richer, and a full cup of it bakes into a greasy layer. The double cream substitute and the heavy cream substitute are there if the carton is missing. A substitute that is mostly milk will not brown the same way, and the potatoes need a little extra time.

Slice them the same thickness

3 mm is thin enough to cook through in an hour and thick enough to stay a slice. A mandoline is faster and more even. A knife is fine if you check the first few slices against each other. Rinse only if the slices are caked in dirt. Washing all the surface starch away makes a gratin that never binds. A little starch is what turns the cream silky instead of watery.

Warm the cream before it hits the dish. Cold cream onto cold potatoes spends the first 20 minutes of the oven just coming up to temperature, and the center undercooks. Do not boil it. Boiled cream can separate before the potatoes are in the pan.

Cover, then uncover

Foil for 45 minutes steams the potatoes so they soften. Uncovered time is only for color. If the top is brown and the center still resists a knife, cover it again and give it 10 more minutes. Color is not doneness.

Rest 10 minutes. Straight from the oven, the cream is thin and the portions slump. After the rest, the slices lift out in stacks. Leftovers reheat, covered, at 160°C until hot. They will not be as loose as the first night. A spoon of milk on top before reheating helps.