Ingredients
2 sweet white onions, cut in half 
3 tbsps oil 
4 tsps salt 
1 tbsp sugar

Method
Without trimming the onions, cut in half through the root ends.  You need the onions to stay together so very very carefully peel off the outer layers then cut off the very top and the very bottom of the root so it is neat. Heat a frying pan with a tbsp of oil in a pan that can go into the oven on a high heat.  On the cut side put some salt and a little sugar. Add the onions to the pan, cut side down and reduce the heat to medium low and cook until the onions are charred (15 minutes or so). Put into the oven for half an hour until soft. 

Take out the onions from the oven and if you want them to be petals then leave until just cool enough that you can separate each layer into ‘petals’.