Vegetable Boulangère Gratin with Cheese

Preheat the oven to 180°C fan, or 200°C in a conventional oven, or 400°F. Grease a 28 × 20 cm (or equivalent 2-litre) baking dish generously with butter. Using a mandoline or sharp knife, slice the potatoes and celeriac into 2–3mm rounds. Combine the grated Gruyère and Comté in a bowl and set aside.

Place a wide frying pan over medium-low heat and melt the butter with the olive oil. Add the sliced onions and a pinch of salt and cook, stirring occasionally, until deeply golden and jammy — about 25 minutes.

Add the garlic and thyme and cook for a further 2 minutes until fragrant. Remove from the heat.

Spread half the onion confit evenly across the base of the prepared dish. Arrange a single overlapping layer of potato and celeriac slices alternately across the onions, seasoning with salt, pepper, and a little nutmeg as you go. Spread the remaining onion confit over this layer, then continue layering the remaining potato and celeriac slices, seasoning each layer, until all the vegetables are used and the top layer is neat and tightly packed.

Pour the hot vegetable stock slowly and evenly over the layered vegetables — the stock should come to just below the top layer. Press down firmly with a spatula to compact the gratin.

Cover the dish tightly with foil and bake for 45 minutes, until the potatoes and celeriac are almost completely tender when pierced with a thin knife.

Remove the foil, scatter the combined cheeses evenly over the top, and return the dish to the oven uncovered for a further 25 minutes until the cheese is deeply golden, bubbling, and the stock has been fully absorbed.

Remove from the oven and rest the gratin for 10 minutes — this is essential for the layers to set and slice cleanly. Scatter over the chopped parsley and serve directly from the dish.