Mix water and sugar in a bowl until sugar is dissolved.
Put flour, yeast and salt in another large bowl.
Add in the sugar-water and oil and mix with a spatula until combined (it doesn't have to be perfectly smooth). Cover the bowl with a wet towel and allow to rest for 10 minutes.
Then form a ball, no dry streaks should remain in the bowl. Sprinkle the dough with 3 tablespoon of olive oil and turn it a couple of times to make sure it's covered with oil (so that it doesn't dry out while resting).
Cover the bowl with a wet cloth and allow it to rise at room temperature for 90 minutes (or until doubled in size).
Preheat the oven to 180 °C (340 °F) and line a Ø 23 cm (9 inches) pie form with baking paper.
Place the dough into the form, spread it out carefully with your fingers (you want as much air as possible to stay in the dough!).
Poke the dough with your fingers, sprinkle with olives and onion slices, drizzle with olive oil and let it rise for another 30 minutes.
Bake in the middle of the oven for 25 minutes.
Remove the focaccia from the oven, drizzle it with a little olive oil and let cool briefly.