Pear and roquefort tarte tatin

Method

1. Fill a bowl with water and add the lemon juice. Peel, core and quarter the pears, adding them to the lemon water as you work (this will stop them going brown).
2. Flour a large sheet of baking paper, then roll out the pastry into a 30cm circle (3-4mm thick). Prick it all over with a fork, then transfer to the baking paper and into the fridge.
3. Add half the butter to the frying pan and put over a low-medium heat. Once melted, add the leeks, thyme, bay leaf and a good pinch of salt. Cook for 15 minutes, stirring occasionally, until the leeks are soft. Discard the thyme and bay leaf, season well with salt and pepper, then transfer the leeks to a plate.
4. Heat the oven to 190°C/170°C fan/gas 5. Put the pan back over a medium heat. Add the rest of the butter, the vinegar and sugar and stir until the sugar dissolves and the liquid is slightly syrupy. Add the pears and use tongs to turn them until they’re coated in the syrup. Arrange them as best you can in a concentric circle, rounded-side down. You want them to be quite packed in, as they’ll shrink a little. Cook gently for 5 minutes, then remove from the heat. Carefully cover the pan with foil (or a lid) and bake in the oven for 15 minutes.
5. Remove the foil/lid, then spread the leeks evenly over the pears. Put the chilled pastry disc on top, gently tucking the pastry edges down the sides. With a small, sharp knife, make 3 evenly spaced 3cm cuts in the top of the pastry to let the steam out.
6. Bake for 30-35 minutes until the pastry is golden. Let the tart cool in the pan for 10-15 minutes before running a table knife around the edge to loosen the pastry. Put a plate over the top and carefully invert the tart onto the plate. Just before serving, crumble the roquefort over the top and sprinkle with the hazelnuts. Finish with some thyme leaves.