French Potato Pie

There’s no denying that the rich combination makes for a dish that is très décadente. But if you love silky potatoes, buttery pastry, and rich cream, it’s a perfect dish for when you’re in the mood to indulge, and I couldn’t wait to add my own version to the Cook’s Illustrated canon.

I started with the crust. The vast majority of recipes specify puff pastry, and although I found that puff pastry was flaky and delicate on the top of the pie, it was prone to getting soggy underneath the moist filling. Stamm calls for pâte brisée (aka pie dough), as did Julia Child and Simone Beck in Mastering the Art of French Cooking (1961). Sure enough, the thicker, sturdier dough was better at staying crisp on the bottom, especially when I used my colleague Andrea Geary’s foolproof all-butter pie dough.

I built my pie by tossing thin-sliced Yukon Golds with salt, pepper, nutmeg, garlic, and parsley; shingling them over the bottom crust with layers of sliced onion; and enclosing the pie with a vented top crust that I glossed with a beaten egg. After baking, I followed the unusual—albeit common here—step of pouring the cream (I preferred its pure taste to that of tangy crème fraîche) into the vent until the potatoes absorbed as much as I thought they could hold.

As a potato-pie novice, deciding how much cream to drizzle in was a guessing game. It was also tricky to know when the potatoes were tender: Poking a knife into the few slices exposed by the vent didn’t tell me if those deep inside were also done. Finally, the onion didn’t always cook through.

I tackled the potato and onion cookery first. Some recipes call for parboiling the potatoes, others for salting the onions to jump-start the softening process. I found that just a few minutes of boiling was enough to start tenderizing the potatoes so that they were guaranteed to cook through in the oven. And letting the onions comingle with salt drew out their moisture and softened them up perfectly.

Getting the cream into the pie—evenly and in the ideal quantity—was a bigger challenge. The postbake pour was a nifty technique, but it had downsides: Once the potatoes were cooked, they packed together pretty tightly, which prevented the cream from spreading evenly. And because I couldn’t tell how far down the cream had traveled, it was easy to overload the pie, which resulted in a soupy mess upon slicing.

Some recipes call for adding the cream to the potatoes prior to baking the pie, so I decided to try that, with the goal of adding enough to fully coat the potatoes without turning the filling runny.

I mixed 1 cup of cream with 2 pounds of peeled, sliced, parboiled potatoes; filled the crust; and baked the pie. I could cut this sample into neat wedges, but it begged for more cream. Yet when I incorporated another 1/4 cup, the filling was too loose. So I added baking soda to the potato cooking water. Simmering the potatoes in a mild base would cause their exteriors to degrade and release starch. If I then continued to parcook the potatoes in the cream, the starch would thicken it, helping produce intact slices of pie.

I parboiled the potatoes in the soda-laced water for 1 minute (I didn’t want to overdo it since they would also cook in the cream); drained them; and returned them to the pot along with the salted onion, seasonings, and 1¼ cups of cream. I stirred the mixture over gentle heat until the cream began to thicken, about 5 minutes. It worked like a charm: Once baked, this pie that I adore had just the right creamy richness, and it sliced beautifully.