Add the flour, water, yeast and salt to a large bowl that will fit in a fridge. Mix the ingredients with a silicone spatula until you cannot see the white, floury patches any more.
Using the spatula, take the dough out of the bowl for a moment. Pour 2 tablespoons of oil into a bowl and put the dough back there, turn it so that it is immersed in olive oil on each side.
Cover the bowl tightly with a cling film or lid, and put in the fridge for 8 – 24 hours.
After this time, cover a 20x30cm baking tin with one spoon of olive oil and transfer the dough.
Stretch the dough with your fingers, making dents in it, and leave covered in a warm place for 1.5 – 3 hours to rise.
Approx. 15 minutes before you want to bake your focaccia, preheat an oven to 250 degrees.
Gently make a few holes in the dough with your finger, pour the remaining 1 tbsp of olive oil, sprinkle with salt and rosemary, and put in a hot oven. Bake for 16-20 minutes until focaccia is nicely brown like in the pictures.