In a jug, whisk together the water & yeast then leave to go frothy, 5-10 minutes.
Give the activated yeast another whisk then add into the bowl of a stand mixer along with the flour. Mix with the dough hook until just combined then cover & leave to autolyse for 30 minutes.
Next, add in the salt & olive oil & mix on a medium speed until the dough passes the windowpane test. This will take around 15 minutes.To perform the window pane test, stretch a small amount of dough between your fingers. If the dough stretches thin enough so that you can see your fingers through it without tearing, it's ready. Transfer the dough to a lightly oiled glass bowl & leave at room temperature for 1 hour. During this hour, coil fold the dough every 15 minutes.
Cover the bowl with clingfilm & place in the fridge for at least 24 hours (no more than 48).
Grease & line a 9”x13” baking tin then pour in 2 tbsp of olive oil.
Tip the dough into the tin then gently stretch towards each corner, but don’t worry if the dough doesn’t completely fill the tin at this point. Cover loosely with clingfilm & leave to rise at room temperature until roughly doubled in size, around 2-3 hours.
Preheat an oven to 220°c/200°c fan (428°f/392°f) around 30 minutes before you cook the focaccia.
Once the dough has risen, pour a good amount (around 2 tbsp) of olive oil over the focaccia & use your fingers to dimple the dough all over, making sure that the dough reaches all corners of the tin.
Sprinkle the focaccia with a generous amount of sea salt then bake until a deep golden brown. This will take 25-30 minutes.
Once baked, brush the focaccia with 2 tbsp of olive oil, leave to cool in the tin for 20 minutes then transfer to a cooling rack to cool completely before serving.