The day before you want the focaccia, mix the flour, water and yeast together until well blended then leave covered with clingfilm on the worktop for 12-24 hours. I do this in my stand mixer bowl so it's ready to go tomorrow.

When the poolish has risen and is very bubbly, 12-24 hours later, fit the bowl to your stand mixer and using the whisk attachment, whisk the water and oil into the poolish. When well combined, switch to the flat beater attachment (the dough hook is no use for such wet doughs) and sprinkle over the flour, salt and yeast then turn the mixer onto low and beat for 10 minutes until it is elastic and starting to leave the sides of the bowl.

Drizzle a little more oil over the top of the dough and scoop it up into a ball, placing the oiled side of the dough down before covering.  Allow to prove until doubled in size - this may take 2 hours in a cold kitchen so leave it somewhere snug.

When proved, take a baking tin lined with baking parchment and oil it lightly. Using a dough scraper or spatula, coax the soft and bubbly dough onto the baking parchment without deflating it. With oiled hands underneath the dough, slowly and carefully stretch it out to an even thickness of about 1 inch. Cover with oiled clingfilm - try to seal the edges but leave space for expansion underneath the cling. Allow to rest for 40-60 minutes somewhere warm whilst the oven is preheating to 220˚C. 

The dough is ready to bake when it is starting to show a few big bubbles under the surface and has risen in size by about 50%. Drizzle the remaining 1 tbsp of oil and dimple the surface lightly with your fingers. Sprinkle with the sea salt and rosemary and bake for 25 minutes until well browned and risen. It should sound hollow when tapped in the centre. Cool for 5 minutes on the tin before transferring to a rack until cool enough to slice and eat.