Add flour, water, yeast, and salt to a bowl and mix with a spoon or hand until you have a very wet and sticky dough
Cover and leave to rise at room temperature until tripled in size and jiggly, or leave overnight in the fridge
Oil the tin and scatter with polenta if desired
Tip the dough into the tin with oiled hands and stretch out to a large square or rectangle
Brush or rub olive oil over the dough
Cover with a tea towel or oiled cling film and leave to prove for about an hour (or overnight in the fridge) until puffy and risen
Preheat the oven to 200°C fan
Mix 2 tbsp olive oil and 1 tbsp water and brush over the dough
Using your fingers, push into the dough to feel the bottom of the tin for the distinctive focaccia appearance
Sprinkle flaky sea salt and add extras like garlic, rosemary, or olives if desired
Bake for 20 minutes at 200°C (add water to the oven for steam), then turn down to 180°C for a further 10-15 minutes until golden and crispy
Remove from oven and drench with olive oil and water emulsion while hot
Let cool for 10 minutes before serving
