Mix the flour, salt, and yeast together in a large bowl
Add the warm water and mix together into a rough dough
Prove for about 1 hour, folding every 15-20 mins.
Put the olive oil, garlic, and flaked sea salt into a separate large bowl, and add some picked rosemary or sage leaves
Form the dough into a ball, place it into the bowl with the olive-oil mixture and roll it around to coat it, before leaving it in the mixture to prove for another hour
Gently tip the dough and oil mixture into a high-sided baking tray about 20 x 30 cm, and use your fingers to prod and push it reasonably evenly across the base of the tray right into the corners. If it won’t spread out all in one go, rest the dough for 5 mins and try again. Make sure any rosemary or sage leaves are tucked underneath the dough and not on the top surface where they will burn when baked.
Leave to prove in the baking tray for another 45 mins to 1 hour.
Preheat the oven to 200C
Make holes in the dough by pressing your finger all the way down to the base of the tin every 4-5 cm, and decorate by putting tips of rosemary sprigs into some of the holes
Bake for about 20 mins until the top surface is lightly browned
Lift the focaccia out of the tin onto a cooling rack and sprinkle with flaked sea salt.