Peel the onions and cut into thick slices around 1.5 centimetres or half an inch.
Put the onions in a shallow tray and pour over boiling water. Leave them to soften for 10 minutes undisturbed.
Cut the pastry to 25 centimetre width circle and use a fork to make venting holes .
Heat the butter and oil over a medium heat with a few sprigs of thyme.
Add the onions to the pan, wider cut side down, and cook over a medium to low heat for a further 10 minutes.
Sprinkle over the sugar and allow it to dissolve, then add in the salt, pepper, brandy and vinegar and let the mixture bubble for a minute. Remove the pan from the heat.
Preheat the oven to 200 C / 400 F /180 FAN / Gas Mark 6.
Place the pastry over the onions and roughly tuck the pastry around.
Cook for 30 minutes until puffed and golden.
Leave the onion tarte tatin to cool for a few minutes then place a plate over the pan and invert, to turn out the tart.
Notes
The nutritional value is calculated as if the whole pastry sheet is used. There will be some leftover.