Nico Moretti's Cherry tomato and red onion focaccia

Dough

• 20 g extra virgin olive oil, plus extra to grease
• 400 g lukewarm water
• 1 ½ tsp dried instant yeast
• 500 g baker's flour
• 2 tsp sea salt

Topping

• ½ red onion, cut into squares (2 cm), layers separated (24 pieces)
• 12 mixed cherry tomatoes, cut in half
• 1 tbsp fresh rosemary, leaves only
• 3 tbsp extra virgin olive oil
• 1 tsp sea salt flakes, to sprinkle