Heat 2 tablespoons olive oil in a large saucepan, add the onion and cook for 5 minutes until softened. Stir in the garlic and cook for 30 seconds more. Add the rice and stir around the pan until well coated. Pour over the wine and leave to bubble for 2–3 minutes until reduced by half.
Add the tomatoes and stock, season and bring to the boil. Leave to cook for 10 minutes until most of the liquid has boiled away, then turn down the heat to low, cover and leave for 5 minutes until the rice is soft.
Meanwhile, heat the remaining oil in a large frying pan. Add the prawns and cook for 2 minutes until turning golden. Turn over and carefully pour over the ouzo, if using. Cook for 2–3 minutes more until the prawns are cooked through and the ouzo has reduced. Season to taste. Transfer the rice to plates, arrange the prawns on top and then scatter over the oregano, feta and chilli flakes to serve.