Pommes Purée Gratinée Recipe

Preheat and prepare

Preheat the oven to 425°F (220°C). Butter a 9x9-inch or similar shallow baking dish (or individual gratin dishes).

Peel the potatoes and cut them into roughly 1/2-inch (1.2 cm) thick slices so they cook evenly. Rinse the slices under cold water and pat dry with a kitchen towel.

Place the potato slices in a large pot and cover with cold water by about 1 inch. Add 1 teaspoon of the salt. Bring to a simmer over medium-high heat, then reduce to a gentle simmer and cook until potatoes are just tender when pierced with a knife, about 8–10 minutes (they should not be falling apart). Drain thoroughly in a colander and let steam-dry for 2–3 minutes.

While the potatoes drain, warm 1 cup of whole milk with 4 tablespoons of butter in a small saucepan or microwave until the butter melts and milk is hot but not boiling. This warmed milk will be used to mash the potatoes for the purée.

Transfer the drained hot potatoes to a large bowl or back into the warm pot. Mash with a potato masher or ricer until smooth. Gradually add the warmed milk and butter mixture, mixing to a creamy purée. Taste and season with the remaining 1/2 teaspoon salt and the 1/2 teaspoon freshly ground black pepper. The texture should be very smooth and spoonable but still thick enough to mound.

Spoon about two-thirds of the potatoes into the prepared baking dish and spread into an even layer about 1/2–3/4 inch thick. Reserve the remaining purée for the top layer.

Once the Mornay sauce is added (see below), spread the remaining potato purée on top, creating a smooth surface or little peaks for browning. Finish by sprinkling the reserved 1 cup grated Pecorino evenly over the top.

Bake in the preheated oven for 20–25 minutes until bubbling and the cheese is lightly browned. If more color is desired, set oven to broil and broil for 1–2 minutes, watching closely to avoid burning.

Remove from oven and let rest 5 minutes. Sprinkle with chopped chives or parsley if using. Serve hot.

Make a roux: In a medium saucepan over medium heat, melt 2 tablespoons butter. When it foams but before it browns, add 2 tablespoons flour and whisk constantly for 1–2 minutes to cook the raw flour taste; do not let it color.

Gradually whisk in 1 cup of warmed whole milk (warm the milk separately to avoid lumps) in a steady stream. Continue whisking until the sauce is smooth and starts to thicken, about 3–4 minutes. Reduce heat to low.

Whisk in 1/2 cup freshly grated Pecorino Romano until melted and smooth. Stir in 1/2 teaspoon salt, 1/4 teaspoon pepper, 1/2 teaspoon Dijon (if using), and a pinch of nutmeg. Taste and adjust seasoning.

Remove the Mornay from heat. If it is very thick, thin slightly with up to 1–2 tablespoons warm milk so it has a spoonable, pourable consistency for layering between the potato purée layers.

Spoon or drizzle the warm Mornay sauce evenly over the first layer of potato purée in the baking dish so it soaks into the surface and helps the layers bind. Then top with the remaining potato purée and proceed to finish with grated Pecorino and baking as directed above.