Brandade de Céleri-rave Gratinée au Fromage

Heat the oven to 200°C (180°C fan). Scrub the celeriac and pierce each bulb several times with a skewer. Wrap the unpeeled garlic cloves loosely in foil. Peel and chunk the potatoes. Measure and warm the milk. Zest and juice the lemon. Grate both cheeses and combine them. Mix the breadcrumbs with a small pinch of salt.

Place the celeriac bulbs directly on the oven rack and set the foil parcel of garlic alongside. Roast for 60–70 minutes, until a skewer meets no resistance at the centre and the skins are papery and darkened.

While the celeriac roasts, boil the potato chunks in well-salted water over high heat for 18–20 minutes, until completely tender when pierced with a knife. Drain thoroughly and return to the dry pan over low heat for 2 minutes, shaking the pan, until any surface moisture has steamed off.

Remove the celeriac from the oven and leave until cool enough to handle. Halve each bulb and scoop the flesh away from the skin with a large spoon, discarding the skin. Squeeze the roasted garlic from its papery skin. Pass the celeriac flesh, potato, and roasted garlic together through a potato ricer or fine-mesh food mill into a large, heavy-bottomed saucepan — do not use a blender or food processor, which will make the mixture gluey.

Set the saucepan over low heat and stir the mash continuously with a wooden spoon for 3–4 minutes to drive off further moisture, until the mixture pulls away cleanly from the sides of the pan.

With the pan still over the lowest possible heat, begin adding the olive oil in a thin, steady stream, stirring vigorously and continuously after each addition, as though making a mayonnaise. Alternate with small additions of the warm milk, incorporating fully before adding more, until the brandade is smooth, glossy, and just holds its shape. Beat in the lemon zest, 1 tablespoon of lemon juice, white pepper, and parsley. Taste and adjust the salt.

Heat the grill (broiler) to its highest setting. Lightly oil a 22–24 cm gratin dish or four individual 300 ml gratin dishes. Spread the brandade evenly into the dish or dishes, smoothing the top with the back of a spoon. Scatter the combined Gruyère and Comté evenly over the surface, then sprinkle with the breadcrumbs and dot with the butter pieces. Drizzle lightly with olive oil.

Place under the grill for 8–10 minutes, until the cheese is deeply golden and bubbling at the edges and the breadcrumb crust is crisp. Watch closely — the topping can colour from golden to burnt in under a minute under a powerful grill. Rest for 3 minutes before serving.