Use a baking tray; grease tray. Sprinkle yeast onto water to dissolve. Place flour in a large bowl and stir in the salt. Make a well in the centre; pour in yeast mixture and 3 tbsp oil. Mix flour into liquid ingredients, using a wooden spoon first and then your hands, to make a soft, slightly sticky dough.
Turn dough out onto a lightly floured work surface; knead for about 10 minutes or until smooth and elastic. Keep dough moving by turning, punching and folding it to prevent it from sticking. Sprinkle a little more flour onto work surface, if necessary; be careful not to add too much or dough will become dry and loose its elasticity.
Shape dough into a ball and place it on the baking tray. Roll out dough (or push it out with your hands) into a round approximately 20cm in diameter and 2cm thick. Cover tray loosely with a cloth kitchen towel; tuck ends underneath. Leave in a warm place for about 45 minutes or until dough has doubled in thickness.
Preheat oven to 230 degrees C. Remove the towel. Pour a little lukewarm water into a cup, then dip your fingers into the water and press them into the risen dough, making deep indents over the whole surface; wet your fingers each time so that the top of the loaf is very moist. Brush remaining oil over the bread and sprinkle on the coarse sea salt.
Bake focaccia for 15 minutes or until golden brown. Transfer to a wire rack; cool for 15 minutes. Wrap the bread in a clean towel to soften the crust as it continues to cool. Serve warm or cold. Focaccia can be kept in a polythene bag for up to 2 days.