Place the flour, salt, sugar, yeast, water and oil in a stand mixer bowl. If you don't have one just place it in any bowl.
Mix everything up a bit with the hook attachment and then knead on high speed for 7 minutes. If you're not using a stand mixer, knead by hand for 15 minutes.
Place a bit of olive oil in a clean bowl and let it proof covered with a kitchen cloth. Let it proof for an hour or until it doubles in size.
Pour olive oil all over an oven tray. Divide the dough in 6 and stretch them out on the tray using your fingers in oval shapes. Make sure that they have about 3 fingers between each other so they can proof and bake without sticking. Cover them again and let them proof for another hour or until doubled in size.
Dampen your fingers with olive oil and gently make holes in the focaccias.
Drizzle them with olive oil and place some onion and mozzarella cheese on each one. The eggplant slices go at the centre. You can do one or two slices. Finish with a bit of coarse sea salt.
Bake in a preheated oven at 230C / 446F for 10-15min or until golden.