Gratin Dauphinois (Potatoes Baked Slowly in Cream)

Variations

• Gratin savoyard: the Savoie version, made with stock instead of cream and topped with Gruyère. The difference between dauphinois and savoyard is exactly that: cream versus stock, no cheese versus cheese. What is usually served as "dauphinois with cheese on top" is really this (stocks and broth).
• On putting cheese on it: French households do it too. It is not in the traditional recipe though, and it buries the taste of the cream. If you are going to, scatter it very thinly for the last ten minutes (dairy and heat).
• Pommes Anna: potatoes and butter only. With no cream, starch and butter are all that hold it, and it turns out as a round cake. Same family as Switzerland's rösti, which also builds a shape from nothing but potato.
• American scalloped potatoes: made with a béchamel (flour, butter, milk) and cheddar. The flour does the thickening, so rinsing the potatoes does not matter and it fails less often — but it tastes of something else (thickening sauces).
• Other vegetables: courgette, cardoon, celeriac and cauliflower all take the same treatment. None of them brings starch, though, so the cream splits easily — mix in some potato, or use cheese to hold it.
• Making it ahead: baked a day early and chilled, it is actually easier to cut. Reheat for 20 minutes at 160°C. Leftovers are at their best cut into squares and fried in butter on both faces (reviving leftovers).
• What it goes with: in France it is the accompaniment to roast meat. Other ways potatoes fill that place here are mashed potatoes and German potato salad; for the same idea of setting cream and eggs in the oven there are quiche, lasagna and shepherd's pie.

Tips

• Do not rinse the potatoes. This is the dish. Most potato recipes tell you to soak the slices to wash the starch off. Here it is the reverse: with no flour, egg or cheese, potato starch is the only thing holding the cream. Rinsed, the cream sinks and the whole thing bakes in separated layers.
• Use cream at 35% fat or above. Made with milk alone, the casein tightens in the oven and the whey separates into a puddle. More fat means more stability under heat, which is the basic behaviour of dairy (dairy and heat).
• 150°C, low and long. Cream splits when it boils. Too hot an oven and fat rises at the sides of the dish and the top scorches while the middle is still hard (what your oven is doing, dry heat and moist heat).
• Do not skip the ten minutes in the pan. You can put raw slices straight into the oven, but dissolving the starch on the hob first cuts the failure rate a lot and shortens the bake.
• Even thickness matters. Mix 2mm and 5mm slices and the thin ones collapse while the thick ones stay hard. A mandoline is the one tool that earns its place here (knife skills).
• Dissolve the salt in the cream. Sprinkled between layers it lands unevenly, and potato is slow to take salt. In the liquid, it distributes over the whole hour (salt and seasoning).
• Cream only to just below the top slices. Fully submerged they poach and the surface never colours. The level drops as the potatoes drink it in.
• Rub the garlic, then throw it away. Chopped garlic catches in dairy over an hour and leaves bitterness and a texture you notice. Only the scent is wanted.
• Raise the heat at the end for the crust. The word gratin refers to that browned surface, and getting there this way is not the same as baking hot from the start (browning).
• Rest it 15 minutes out of the oven. The most commonly skipped step here and the most visible one. Hot, it runs; cooling, the starch sets and it holds an edge (serving temperature).
• Very little nutmeg. A long-standing partner to dairy and potato, but a fraction too much and it covers the whole dish. One pinch is the ceiling.
• Cheese makes it a different dish. It is good with it, but that is the savoyard side; taste it once without, to find out what cream and potato do on their own.