Preheat the oven to 200ºC/400ºF/gas 6. Peel and finely chop the red onions. Place a 25cm x 30cm roasting tray on a high heat on the hob, pour in 1 tablespoon of olive oil, add the onions and fry for 4 minutes, or until soft and sweet, stirring regularly. Meanwhile, place half the saffron in 600ml of boiling kettle water. In a bowl, cover the remaining saffron with 1 tablespoon of boiling water, steep for 10 seconds, then mix with the yoghurt and put aside.
Stir the tomato paste, rice and a pinch of sea salt and black pepper into the onion tray, then pour in the saffron water and bring to the boil. Once boiling, carefully transfer to the oven for 15 minutes, or until the rice has absorbed all the liquid, fluffed up beautifully and is golden and crisp on top.
Spoon the saffron yoghurt over the rice, drizzle it all with 1 tablespoon of extra virgin olive oil, fork and mix it all together, and dish up.