Place the flour, salt, yeast and chopped rosemary into a large bowl. Use a wooden spoon to mix the ingredients.
Add the olive oil and water. Gently stir with your hand or a wooden spoon to form a dough. This will result in a very sloppy and sticky dough.
Lightly oil a clean bowl and tip the dough into it and cover with cling wrap. Allow the dough to rest for 30 minutes.
Stretch the dough by hand in the bowl, tuck the sides into the center, turn the bowl 90 degrees and repeat the process for about 20 seconds. Cover the bowl again and allow it to rest for another 30 minutes. Repeat this process once more.
With the last stretch and fold, cover the bowl and leave the dough at room temperature to proof until doubled in size, roughly 1 – 2 hours.
Line a large baking tray (25x35cm) with baking paper. Tip the dough out of the bowl and flatten the dough onto the prepared tray, pushing to the corners, cover with a large plastic bag, making sure it does not touch the top of the dough, then leave to prove for one hour.
Preheat the oven to 220C. Drizzle the focaccia with oil, sprinkle with fine sea salt then bake in the oven for 20 – 25 minutes.
When cooked, drizzle with a little more olive oil and serve hot or warm.