Preheat the oven to 200°C/fan180°C/gas 6.

In a saucepan, bring 200ml vegetable stock to the boil with 2 garlic cloves, lightly crushed.

Remove from the heat and stir in 200ml double cream.

Set aside to cool and infuse.

Heat 1 tbsp olive oil in a pan and gently fry 1 large onion, chopped, for 10 minutes, until very soft.

Peel and quarter 1 large celeriac, then slice to 3mm thickness.

Put in a bowl with 3 medium potatoes, sliced to 3mm thickness.

Toss with 1 tbsp plain flour, 1 tbsp chopped fresh flat leaf parsley and some seasoning.

Butter a 1.2-litre ovenproof dish and layer up the celeriac, potatoes and the softened onion.

Strain the cream mixture (discard the garlic) into a jug and pour over the gratin.

Bake for 1-1¼ hours, until golden brown and bubbling and the potatoes are cooked through.