To make the dough, add the lukewarm water, sugar and active dried yeast into a small bowl or jug. Stir and let sit for a few minutes for the yeast to bloom.
Add the flour and salt into a large bowl and mix.
Pour the water into the flour and mix with a wooden spoon until somewhat combined. Now, transfer the dough onto a flat surface.
Add some olive oil into a small dish. Knead the dough for about 5 minutes. The dough will be quite wet and sticky. Whenever you feel the need, dip your fingers into the olive oil - this will make the kneading easier.
After 5 minutes, give the dough a rest for 10 minutes. Then, knead it for another 5 minutes, using the olive oil whenever the dough gets too sticky.
Now the dough should be reasonably smooth and a lot less sticky then what you started with. Transfer the dough back into the large bowl and cover the bowl with a silicon cover or cling film.
Place the dough into the fridge and leave it there overnight or for at least 8 hours.
The next day, remove the bowl from the fridge.
Grease your baking dish or tray with about 1 tbsp of olive oil and then transfer the dough onto the dish. Spread it out. It will bounce back at this point, don't worry. Let it rest for 20-40 minutes (20min if it's a hot day, 40min if it's cold in your kitchen).To see if the dough is ready for the next step, spread it out once again. If it doesn't shrink back, it's ready.
Now, grate the garlic cloves and mix the grated garlic with olive oil in a small dish. Pour the garlic/oil mixture on top of the dough and spread it evenly across while also making little dimples by pushing your fingers into the dough.
Sprinkle the rosemary leaves on top of the dough.
To make the brine, mix the salt with the water in a small dish. Pour the brine on top of the focaccia dough.
Let the dough rest for another 20-30 minutes while preheating the oven to 220°C fan.
Finally, sprinkle a little flaky salt on top of the dough (optional) and bake the focaccia for 25-27 minutes at 220°C fan.