Gather all your ingredients and equipment before you get started! 

The day before, prepare the levain by mixing the sourdough starter, water, and bread flour until well combined. Cover and leave at room temperature overnight, or for about 10 to 12 hours, until it has doubled in size and is bubbly.

In a large bowl, combine the water and ripe levain, stirring until the levain is mostly dissolved. Add the bread flour and mix until no dry flour remains. Cover and allow the dough to rest for 20 minutes.

Sprinkle the salt over the dough and drizzle in the olive oil. Mix by hand until everything is fully incorporated. The dough will be very soft and sticky.

Cover and let the dough rest for another 20 minutes. Then perform one set of stretch and folds by gently lifting the dough from one side of the bowl and folding it over itself. Rotate the bowl and repeat until all four sides have been folded into the centre. Cover and let the dough rest for another 20 minutes.

Repeat the stretch and folds one more time. Transfer the dough to a generously oiled half-sheet pan and gently stretch it toward the corners. It does not need to reach the edges completely at this stage.

Cover with a clean dish towel and allow the dough to proof for 60 to 90 minutes, or until it is noticeably puffy and has spread to fill the pan.

Preheat the oven to 450°F.

Using your fingertips, gently dimple the surface of the dough. Drizzle generously with olive oil and sprinkle with the chopped rosemary, flaky salt, and cherry tomatoes, if using.

Bake for 22 to 28 minutes, or until the focaccia is deep golden brown and crisp around the edges.

Allow the focaccia to cool in the pan for about 15 minutes before slicing and serving.