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Heat the oven to 200C/180C fan/gas 4. Unravel the puff pastry sheet, removing the parchment. Cut a 25cm circle from the pastry, using an ovenproof frying pan as a template. Put the pastry circle on a tray and chill until needed. Melt the butter in the same pan over a medium heat. Once foaming, arrange the onions in concentric circles in the pan, packing them in tightly, and reduce the heat to low. Season with a pinch of salt and cook for 10-15 mins until the onions are beginning to caramelise. Keep an eye on them so they don’t burn.
Add the vinegar and sugar, and turn the heat down to medium-low. Cook for 5-8 mins more until a sticky glaze has formed in the base of the pan and the onions are deeply caramelised. Remove the pastry from the fridge and put it on top of the onions, tucking it in around the edge. Prick it all over using a fork, then transfer to the oven and bake for 20-25 mins, or until the pastry is golden brown and puffed up.
Meanwhile, make the salsa verde. Combine the herbs and capers in a large bowl, then add the lemon juice, vinegar and olive oil. Season with salt, pepper and a pinch of sugar.
Remove the tarte tatin from the oven and leave to cool for 5 mins. Put a large plate over the pan and invert the tarte tatin onto it, leaving the pan in place (you’ll need to use oven gloves as the pan will still be hot). Leave the pan upside-down on the plate for a few minutes more (this will help the tart release from the base naturally), then remove the pan and spoon over the salsa verde before slicing into wedges to serve.