Vegan Vegetable Boulangère Gratin

Peel and slice the potatoes, celeriac, and parsnips to 3mm using a mandoline or sharp knife, keeping each vegetable in a separate pile. Halve and thinly slice the onions. Mince the garlic. Pick the thyme leaves. Measure the stock and warm it in a small saucepan.

Preheat the oven to 180°C fan, or 200°C in a conventional oven, or 400°F. Heat 2 tablespoons olive oil in a wide, heavy-based pan over medium-low heat. Add the onions and ½ teaspoon salt and cook, stirring occasionally, for 20–25 minutes until deeply softened, golden, and sweet — they should be collapsing and jammy, not browned.

Add the garlic and thyme to the onions and cook for 2 minutes until fragrant, then remove the pan from the heat.

Rub a 30 × 20cm gratin dish with the remaining 1 tablespoon olive oil. Spread half the onion mixture across the base. Lay the potato slices in an overlapping shingled layer across the onions, season lightly with salt and white pepper, then layer the celeriac in the same fashion, season again, then finish with the parsnips in a final shingled layer. Spoon the remaining onions evenly over the top. Tuck in the rosemary sprigs and bay leaf.

Pour the warm vegetable stock slowly and evenly over the entire dish — it should come about two-thirds of the way up the vegetables without submerging the top layer.

Cover tightly with foil and bake for 50 minutes until the vegetables are just tender when pierced with a knife tip.

Remove the foil and bake for a further 30–35 minutes until the top is golden, the edges are lightly crisped, and the stock has reduced to a glossy, clinging fond around the layers.

Remove the rosemary and bay leaf. Rest the gratin for 10 minutes before serving, then scatter with chopped flat-leaf parsley.