In the bowl of a stand mixer fitted with the dough hook, place the sourdough starter and the water. Using your fingers or a silicone spatula, mix the two together until the starter in quite evenly distributed. Add the flours and the salt. Lower the dough hook and mix on medium speed until the dough comes together and forms a shaggy, rather wet ball (around 5 minutes). Turn the speed to low, add the the olive oil and mix for a further five minutes. The dough will not look smooth and glossy yet, so don’t worry.
Grease a large Tupperware tub with a splash of olive oil and transfer the dough to the tub. Grab one side of the dough, pinch and stretch it up and over to the centre of the dough. Rotate the Tupperware 90 degrees and repeat: pinch and stretch the dough into the centre. Repeat two more times until you’ve made a total of four folds in the dough. Cover the tub with a damp tea towel and place in an ambient temperature of around 20-22c. Leave the dough to “bulk ferment” for a total of four hours, repeating the four folds another four times every half hour for the first two hours.
Meanwhile, grease a large, high sided brownie pan (around 23 x 33cwith a good drizzle of olive oil.
After the first two hours of fermenting and folding are done, transfer the dough to the oiled pan, cover and leave for the remaining two hours.
Once the dough is proved, place the entire pan of dough into a plastic bag or wrap with cling film and place in the fridge overnight or for a minimum of 8 hours.
The next day, or after 8 hours have passed, remove the dough from the fridge and place on the kitchen counter to come back to room temperature for an hour.
Once this hour is done, the second prove begins. This can take between 1 and 2 hours depending on the temperature of the room and the activity of your starter. Just keep an eye of the dough after the first hour of proving - it should become very bubbly and gassy.
About half an hour before your dough is ready, preheat the oven to 230c and prepare your toppings.
Once proved and nicely bubbly, wet your fingers and dimple the dough all over, making sure you push all the way down to the bottom of the pan.
Drizzle over the topping olive oil and sprinkle the flaky sea salt. Top with the herbs, olives, garlic and cherry tomatoes then place in the oven. Bake for 25 minutes, rotating after 15 mins.
Remove from the oven and leave to cool in its tin for 15 minutes. Carefully remove the bread from the tin once cool enough to handle and place on a wire cooking rack.
Focaccia tastes best warm within a few hours of baking, but you can also wrap in tin foil and store for a day or so. Enjoy!