Add the water and yeast to the Thermomix bowl and heat for 1 min/37°C/speed 1.
Add the flour, olive oil and salt. Mix for 10 sec/speed 6, then knead for 2 min/dough function.
Turn the dough onto a lightly floured surface and shape into a ball. The dough may be slightly sticky, so lightly flour your hands if needed. Place in a lightly oiled bowl, cover, and leave to prove for 1 hour or until doubled in size.
Turn the dough onto a lined baking tray and stretch it out to roughly 20 cm × 28 cm. Cover and leave to rise again for 30 minutes.
Preheat the oven to 220°C/425°F (200°C fan).
Press dimples into the dough with your fingertips. Mix 1 tablespoon water with 1 tablespoon olive oil and brush over the dough. Sprinkle with sea salt and rosemary if using.
Bake for 20 minutes or until golden.
Drizzle with a little extra olive oil before serving, if desired.
Notes
Focaccia is best eaten fresh, but leftovers can be frozen. The dough can be made ahead of time and kept in the fridge for a few days. When it's time to bake, let it rise for 30 minutes before baking.