Mix the yeast, water and sugar and set aside for 10 minutes.
Add the flour to a stand mixer bowl, pour the yeast mixture and process into a dough. Add the salt and knead for 5 minutes. Cover the bowl and let the dough rise for 40 minutes.
Knead the dough manually for a minute, then cover and let rise for another 40 minutes (repeat one more time after this).
In a medium bowl, pour a little olive oil and place the dough. Pour a tablespoon of olive oil on the dough, cover well and let it rise in the fridge for 20-24 hours.
Day 2:
Take out the dough from the fridge 2 hours before working with it.
Line up a medium size round/square sheet pan with parchment paper, drizzle some olive oil and add the dough. Cover and let it rise for 45 minutes.
Preheat the oven to 430 °F (220 °C).
Uncover the dough and drizzle olive oil, Add the tomatoes, mozzarella balls and rosemary and sprinkle with salt.
Bake for 20 minutes, or until the focaccia starts to brown. Remove from the oven, add pesto or basil and serve.