Salt and vinegar jersey royal focaccia

Method

1. In the bowl of a stand mixer with a dough hook attached, combine the flours, yeast, sugar and lukewarm water. Cover and set aside for 30 minutes. Meanwhile, cook half the potatoes in a pan of boiling salted water for 15-18 minutes until tender. Drain and mash the potatoes (leave the skins on), adding the 2 tsp vinegar.
2. Add the salt to the flour mixture with a splash of water to help it dissolve, then  add the mashed potatoes. Knead for 5-6 minutes while drizzling in the 100ml olive oil. Once smooth, glossy and elastic, cover and set aside for 40 minutes.
3. ‘Coil fold’ the dough: use wet hands to scoop under one edge of the dough, lift it up, then fold it over itself. Turn the bowl by a quarter and repeat, then turn and repeat twice more. Cover and set aside to rest for 40 minutes. Repeat this process 3 more times, every 40 minutes.
4. Once you’ve completed the final fold and rest, line the baking tin with baking paper, then drizzle a little olive oil in the base and spread it out with your fingers. Carefully persuade the dough into the tin and gently ease it out towards the sides, so it’s roughly rectangular and of an even thickness – don’t push or pull too firmly. Don’t worry if it doesn’t reach the edges of the tin – it will fill out as it proves. Cover with a clean tea towel and put in the fridge to prove overnight.
5. The next day, take the tin out of the fridge to let the dough come to room temperature (about 30 minutes). Heat the oven to 200°C fan/gas 7. Finely slice the remaining potatoes and put them in a bowl with 2 tbsp vinegar and a good pinch of salt; mix well.
6. Oil your fingers and press them into the dough to create a deep dimple pattern all over the top. Scatter with the sliced potatoes. Drizzle over plenty more olive oil, sprinkle with sea salt flakes, then bake for 30 minutes. Turn the heat down to 180°C fan/gas 6 (don’t open the oven) and bake for another 15-20 minutes until golden. Transfer to a wire rack to cool, but feel free to eat while still warm. Serve with more olive oil and vinegar for dipping.