This is so hearty that you could turn it into a meal by serving it with Leek and Cheddar Welsh rarebit (see here). To save your energy bills, it’s best to cook the potatoes when you are using the oven for something else; you can always keep the cooked potatoes in the fridge for two days until you are ready to make the soup. Whipping cream makes the soup a little bit lighter than double cream would. You can elevate the recipe by adding a poached egg to the bowl, ladling the soup over the top and then adding the chive sour cream. Keeping the egg soft-poached, so it pops when you cut into it, makes a nice surprise at the bottom of the bowl.
1kg baking potatoes (you won’t be able to get this weight exactly due to the random size of the potatoes, but close to 1kg will do; it’s about 4)
50g unsalted butter
1 large onion, finely chopped
2 tbsp chopped chives
100g sour cream
sea salt and freshly ground black pepper
125ml whipping cream, plus more if needed (optional)
whole milk, if needed (optional)
Preheat the oven to 230°C/fan 210°C/gas mark 8. Scrub the potatoes and bake for 1 hour, or until tender when pierced with a knife. When they are cool, carefully peel with a small knife, not a peeler. Keep the skins. Chop the potato flesh roughly; don’t be fussy, you are going to blend it later.
Pour 750ml of water into a pan, add the skins and simmer for 10 minutes. Allow to cool in the pan.
Melt the butter in a separate large saucepan and gently cook the onion until nice and soft, but not coloured. Strain the water from the potato skins through a sieve and add it to the pan with the onion. You can throw the skins away now, there’s not much more you can do with them. Now add the potato flesh and gently simmer for 10 minutes.
Meanwhile add the chives to the sour cream, season and mix.
Remove the soup from the heat and blend with a hand-held blender, being careful of hot splashes. When it is nice and smooth, reheat if needed, then add the whipping cream. Do not simmer or boil once the cream is added, as it could split. Depending on the starch content of your potatoes, you may need to adjust the consistency of the soup with milk.
Serve, adding a dollop of the chive sour cream at the last minute so it sits on top of the soup.