First, make the leaven. Take the sourdough starter out of the fridge and add 100g of the strong bread flour and 100g water (it’s easier to measure water accurately by weight than volume). Mix thoroughly to remove all the lumps – get your hands dirty! Leave in a warm place overnight, covered with cling film or a tea towel.
The next day, weigh out 620g water and pour it into a large metal bowl, then add 100g of the leaven you mixed yesterday (return the rest of the culture to the fridge for future use) and the
yeast. Stir with your hand, then incorporate the rest of the flour and sea salt thoroughly until any lumps of flour are gone. Leave this in the bowl for 30 minutes, then place in the fridge overnight, if you have time.
The next morning, line a large baking tray with parchment and coat with olive oil.Dump the dough out onto the baking tray and gently fold each side into the middle, then flip the entire dough over on itself.Leave in a warm spot, loosely covered, for 30 minutes, then do the same folds as before. Repeat every 30 minutes for about 4 hours.(After a few turns you will notice the volume increase and bubbles start to form on the surface.)
At the 4-hour mark it should have tripled in size and be spreading out. Now gently tease the dough out to the corners of the tray, pulling from the centre underneath the dough while also being careful not to break through it. Try to keep as much air inside as possible, as this is what will help it to rise and have a lovely open, bubbly structure.
Leave the focaccia for a further 30-60 minutes to prove to the height of your tray.
Preheat the oven to 240C/475F/gas mark
Liberally drizzle the top of the focaccia with good-quality olive oil, and then push your fingers down and forwards, to distribute the dough evenly and exaggerate those cavernous bubbles!Top with the onion, tomatoes, Jersey Royals, and rosemary, pushing them firmly into the dough.
Bake for about 25-30 minutes or until golden, then place on a cooling rack and drizzle with more olive oil and a small handful of flaked sea salt.