1. Peel and finely slice all the onions. Place a large heavy-bottomed pan onto a medium-high heat. Once hot, add the olive oil and onions. Season generously with sea salt - the salt will draw out the moisture from the onions and help them to caramelise properly - and stir regularly for two mins. Then turn the heat down to a medium-low heat and cook for about 15 mins. Stir occasionally.
2. After the 15 mins your onions will have reduced significantly in volume and will be starting to caramelise. Turn down the heat to very low and continue to cook for 40 mins (still stirring every so often). As the onions get closer to being fully caramelised they’ll need closer attention so that they don’t burn.
3. Add the chilli flakes and a generous amount of freshly ground black pepper. Stir through and cook for a minute. While that’s cooking - or while you’re cooking the onions in step two - peel and grate your potato using a box grater. Add the grated potato, along with another generous pinch of salt, to the mixture. Stir through. Return the heat to medium and cook for a couple of mins before adding the water. Bring to the boil and then reduce to a simmer. Leave to cook gently for 30 mins.
4. Meanwhile, slice your ciabatta ready for toasting. Do this in a toaster, under the grill or, even better, drizzle each slice with a generous amount of good-quality olive oil and toast in a frying pan over a medium heat.
5. After the potato has been cooking for 30 mins, turn off the heat and leave for 5-10 mins to cool slightly before checking the seasoning. When happy, divide between warmed bowls and serve with the warm ciabatta.
Note: if you’re not vegan, this soup is often served with pecorino and caciocavallo (a type of stretched-curd cheese) so feel free to add some if you like.
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