Caramelised onion tarte tatin

Method

1. Preheat the oven to 200°C/fan180°C/gas 6. Unroll the pastry onto a baking sheet lined with baking paper. Prick all over with a fork, then put in the fridge to chill.
2. Melt the butter in a heavy-based ovenproof frying pan over a medium heat. Ideally it needs to be about 20cm across the base, 25cm across the top and 5cm deep. Add the onion halves, cut-side up, and fry gently for 10 minutes until soft and starting to caramelise, turning them over gently halfway through. Add the sherry and rosemary sprigs, season well, then cover with a lid from another pan or some foil. Transfer to the oven for 20 minutes until the onions are soft.
3. Take the pastry out of the fridge and drape it over the pan, tucking in the edges around the softened onions. Bake for 20-25 minutes until golden and crisp. Remove from the oven and cool in the pan for a few minutes before turning out carefully onto a wooden board or platter. Season and slice to serve.