Prepare the Dough

1. Combine the active sourdough starter, lukewarm water, malt extract (or honey), Vitamin C (if using) olive oil, black tahini, and instant yeast. Mix well until everything is well combined.
2. In a separate bowl, combine the white bread flour, wholewheat bread flour, Kosher salt, and malt extract (if using). Mix the dry ingredients thoroughly.
3. Gradually add the dry ingredients to the wet ingredients while mixing. Mix until a shaggy dough forms.
4. Knead the dough in the stand mixer for about 5 minutes until it becomes smoother. Slowly add the olive oil, to allow the dough to absorb it, until you have a very soft dough that will fall off the hook when you lift it out of the bowl. It will be very soft and relatively sticky.
5. Allow the dough to rest for 30 minutes for an autolyse phase to begin.
6. Knead the dough for an additional 10 minutes in the stand mixer to develop gluten. The dough should become more elastic and less sticky.

Rest and stretch and fold

1. Transfer the dough to a well-oiled bowl. Perform two stretch and fold sessions, with oiled hands, spaced 30 minutes apart. To do this, gently stretch one side of the dough and fold it over the center, then repeat with the other three sides.

Shape into pan

1. Use a standard sheet pan and generously coat it with olive oil to prevent sticking and to develop a crispy crust underneath.
2. After the second stretch and fold session, transfer the dough into the pan and gently it towards the corners. It won’t fill all the corners yet. Cover the pan with oiled clingfilm and let it rest for another 30 minutes.
3. After the resting period, gently stretch the dough into the corners of the pan. Cover it again with oiled clingfilm.

Rest overnight

1. Place the pan in the refrigerator and let the dough rest overnight. This allows the sourdough to develop its unique flavour and the dough to rise to the top of the sheet pan.
2. About 2 hours before baking, remove the pan from the refrigerator and let the dough come to room temperature.
3. Preheat your oven to 450°F (232°C). Once the dough is at room temperature, use oiled fingers to create classic dimples in the dough. These dimples will hold your chosen toppings.

Decorate and bake

1. Sprinkle your preferred toppings, such as flaky salt, rosemary, sumac, garlic, and black sesame, evenly over the dough. Cherry tomatoes or grapes can also be added for extra flavour.
2. Place the pan in the preheated oven and bake for 20-25 minutes on the lower rack, then 5 minutes more on the top rack, or until the focaccia is golden brown and crispy on the edges.
3. Allow the focaccia to cool slightly before slicing and serving