This is such an easy but impressive soup to make. It requires minimum input, but the end result is an intriguing flavour … so maximum Brownie points. Because the celeriac has a taste of celery, the walnut oil pairs very well, while the apples help to add a little acid and balance the flavours – that’s why we use tart Granny Smiths. Make a double batch of walnut oil; it keeps well in the fridge and can be used for salads and other soups. If you have never peeled a celeriac before, cut through the top to make four wedges first; this will make it easier to handle. And use a knife instead of a peeler, as the skin is quite thick.

SERVES 6
50g unsalted butter
1 small onion, finely chopped
1 large celeriac, peeled and chopped
(it may seem a lot but you lose some in the peeling)
1 potato, peeled and roughly chopped
2 Granny Smith apples, peeled, cored and roughly chopped
1 celery stalk, finely chopped
750ml vegetable stock or water
(there is a lot of flavour in this soup so water is OK)
80g walnuts
100ml sunflower oil (you can use olive oil, but we find it overpowering)
125ml whole milk, plus more if needed
125ml whipping cream
sea salt and freshly ground black pepper
In a large saucepan, melt the butter and cook the onion on a low heat, without colouring it. Next add the celeriac, potato, apples and celery. Pour in the stock or water. Depending on the size of your pan you may find that some of the vegetables are sticking out of the liquid – if so, add water to just cover, but don’t go too far as the vegetables will settle down as they cook. Cover and gently simmer for 20–25 minutes until the vegetables are soft.
Meanwhile, make the walnut oil: smash the walnuts a little with the end of a rolling pin just to break them up. Heat a dry frying pan, add the walnuts and dry-fry them quickly, making sure you stir to prevent them burning. This will give a nice roasted flavour to the end result. Turn off the heat and add the oil to the pan – be careful as it may spit – then allow to cool.
Transfer to a clean container until needed (it will keep for 2 weeks in the fridge).
Use a hand-held blender to blend the soup until it is very smooth – it will thicken up because of the starch in the potato.
Now add the milk and cream and reheat, but do not boil as it could split. Depending on how starchy your potato was, your soup may be a bit on the thick side; if so add a little more milk. Season and make sure you use a good few grinds of pepper.
Serve in warmed bowls with a drizzle of walnut oil and some nice nutty wholemeal bread.
We find that some crumbled blue Stilton makes this soup a bit special, especially as it starts to melt into the soup.