Gnocchi Gratin with Vegetarian Nantua Sauce

Measure out and prepare all ingredients: finely dice the shallot and garlic, grate both cheeses and combine them, cut the cold butter into small cubes, and squeeze the lemon juice. Set aside separately.

Place the whole unpeeled potatoes in a large pot of well-salted cold water. Bring to a boil over high heat and cook until a skewer passes through the centre without resistance, about 30–35 minutes depending on size. Drain and return to the pot.

While still hot, peel the potatoes and pass them through a potato ricer or fine drum sieve onto a clean work surface. Spread them out and allow the steam to escape for 5 minutes — this is critical for dry, light gnocchi.

Make a well in the riced potato, add the egg yolk, salt, and nutmeg, then sift over the flour. Using a bench scraper and your fingertips, work the mixture together with minimal kneading until a cohesive dough just forms — overworking develops gluten and toughens the gnocchi.

Divide the dough into 4 portions. On a lightly floured surface, roll each portion into a 2cm-thick rope, then cut into 2cm pieces. Roll each piece over the back of a fork or a gnocchi board to create ridges. Place in a single layer on a floured tray.

Bring a large pot of heavily salted water to a rolling boil over high heat. Cook the gnocchi in two batches: add them and wait until they float to the surface and stay there for 30 seconds. Remove with a slotted spoon and transfer to a lightly oiled baking dish in a single layer.

Preheat your oven to 200°C fan (220°C conventional). Meanwhile, melt the 50g butter in a wide saucepan over medium heat until foaming. Add the shallot and cook, stirring, until completely softened and translucent, about 4 minutes, then add the garlic and cook until fragrant, about 1 minute.

Add the tomato paste, smoked paprika, and cayenne. Stir constantly over medium heat until the paste darkens slightly and catches on the bottom of the pan, about 2 minutes — this caramelisation builds the Nantua-style depth.

Remove the pan from the heat, pour in the Cognac, and return to medium-high heat. Cook, stirring, until the alcohol has almost fully evaporated and the smell sharpens, about 2 minutes.

Pour in the double cream and milk, stir to incorporate all the caramelised bits from the base of the pan, and bring to a gentle simmer. Cook over medium-low heat, stirring occasionally, until the sauce coats the back of a spoon, about 8 minutes.

Remove the sauce from the heat and stir in the lemon juice, salt, and white pepper. Taste and adjust seasoning. Pour the sauce evenly over the gnocchi in the baking dish.

Scatter the combined Gruyère and Parmesan evenly over the surface, then dot with the cold butter cubes.

Bake on the upper rack of the oven until the sauce is bubbling vigorously at the edges and the cheese is deep golden and blistered in places, about 18–20 minutes.

Rest for 5 minutes before serving directly from the dish.