Potato gratin makes me anxious. I'm not anxious to eat it, of course — who would be? — but rather I never feel in control. You're expected to submit to the oven a tray of soupy, raw potatoes and end up with a creamy, tender, caramelised final product. What if the potatoes are undercooked, though? What if it's too wet? It's just too much pressure.
I'm a big fan of scalable formulas in cooking. I have a few stored away already: pasta (1 egg per 100g flour), shortcrust pastry (1 part butter, 3 parts butter, 4 parts flour), and quiche custard (1 egg, 1/3 cup cream, 1/3 cup milk) all work pretty well for the amounts I cook. What's more, by using these over and over I get a sense for what 'just right' looks and feels like, to the point where in some cases I don't need to use them at all. What I need is a formula for potato gratin.
I should warn you now that this entry doesn't contain a tried-and-tested formula, just some preliminary notes. I searched Google for "potato gratin recipe" and looked at a handful of results to see what ratio of potatoes to cream were used. Exclusion criteria were recipes that mixed cheese in with the potatoes (rather than simply on top), recipes with milk or other liquids (because it just gets tricky), and those that didn't include potato weight (seriously, what the hell is '1 average potato'?).
The ratio, r is simply volume of liquid (ml) / weight of potatoes (grams). So for any given weight of potatoes, multiply by r and that's how much liquid to use. Hypothetically.
In addition to learning that I am a huge nerd, we can also see that the ratio tends to be around 0.45. Of course, there's more to potato gratin than potato and cream. There's cooking time, oven temperature, and the shape of the dish. A shallow gratin will cook quicker, a hotter oven will brown the top faster, and a longer cooking time will reduce the cream more. Within the next couple of weeks I hope to test these results using different ratios and dish sizes. Then I'll eat the results and put on 10kg. It'll be awesome.
Meanwhile, here's the recipe for a gratin I made the other night which worked out really well. You'll see that I wussed out and cooked the potatoes in cream first, but hey, Thomas Keller does it so it can't be that bad.
- 725 g désirée potatoes, peeled
- 1.25 cups heavy cream
- 1/2 cup water
- A few gratings of nutmeg
- 1 tsp dijon mustard
- 2 cloves of garlic, halved
- 3 sprigs of fresh thyme (or 1/3 as much dried thyme)
- 5 peppercorns
- 1 bay leaf
- Salt, to taste
1. Use a mandoline or sharp knife to cut the potatoes into thin slices. Wrap up the thyme, bay, peppercorns, and one clove of garlic in cheesecloth, and tie with string to make a neat little package.
2. Combine the cream, water, mustard, and nutmeg in a high-sided frying pan, then add the herb parcel and bring to a very low simmer. Simmer for 5 minutes to begin infusing. Salt the cream very generously — it should taste about the upper limit for what would be palatable, but not ridiculous1.
3. Preheat the oven to 180ºC/350ºF. Add the potatoes to the infused cream (keep the herbs in there as well) and cover. Simmer very slowly until the potatoes are just tender.
4. Rub the sides of a medium-sized baking dish with the cut side of a halved clove of garlic, the discard the garlic. Remove the potatoes from the pan into the baking dish, being careful not to break them as they'll be somewhat fragile. Discard the herb parcel.
5. Pour the cream over the potatoes, and bake until the potatoes are tender and the crust is golden.
(1) Once you add the potatoes it won't be excessively salty.