Peel the celeriac and potatoes, then cut them into bite-sized chunks.

Put them in a pan filled with 7.5 cm - 10 cm salted boiling water, then cook on a high heat for 18-20 minutes, until tender.

Alternatively, if you have leftover cooked potatoes or celeriac, this dish is a lovely way of using them up.

Meanwhile, melt the butter in a pan, add the onion and season generously with salt, pepper and the thyme.

Sweat, stirring, on a medium heat for 10 minutes, then add the garlic.

Drain the roots, then roughly mash them in the pan set over a low heat.

Add the the cream and yoghurt, then mash again.

Add three-quarters of the grated cheese and heat gently, stirring well, for a few minutes.

Stir in the softened onion mixture and season to taste with more salt and pepper, plus a few generous gratings of nutmeg.

Heat the oven to 210C (190C fan)/410F/gas mark 6½, then beat the egg yolks into the vegetables.

In a large, very clean bowl, whisk the egg whites until stiff and immovable, then, using a large metal spoon, gently fold a third of them into the root mash.

Fold the veg mix into the remaining egg whites, taking care not to beat out the air.

Pour the mixture into a large, buttered ovenproof ceramic vessel, sprinkle on the remaining cheese and bake in the centre of the oven for 40-45 minutes; if the top is threatening to brown too much after 30 minutes, turn down the heat to 200C (180C fan)/390F/gas mark 6.

Once the souffle has risen and is smelling glorious, serve at once with the citrus salad.