Caramelised onion tart with walnut and parmesan crust

Method

1. Melt the butter in a large pan, add the onions and toss to coat. Cook over a low-medium heat, partially covered and stirring often, for 1½-2 hours until pale golden and very soft.
2. Meanwhile make the pastry. In a mixing bowl, rub the flour, butter and salt together between your fingertips until the mixture has a sandy texture. Using a dinner knife, stir through the egg and water mixture to bring the dough together, then tip out onto a lightly floured surface and knead very briefly until smooth. Shape into a disc, wrap in cling film and chill for 30 minutes.
3. Heat the oven to 200°C/fan180°C/ gas 6. Roll the pastry out on a floured surface to the thickness of a £1 coin. Use to line a 23cm fluted tart tin Roll a rolling pin over the top to cut off the excess pastry (save for any patching up), then freeze the pastry case for 5 minutes or put in the fridge for 20 minutes. Line the case with foil or baking paper, fill with baking beans/uncooked rice and blind bake for 20 minutes. Remove the beans/rice and foil/paper, prick the base all over with a fork, then return to the oven for 5 minutes or until the pastry is sandy coloured and crisp all over.
4. Add the sherry vinegar and sugar to the onions, then cook for another 15-20 minutes. Taste and season with salt, then leave to cool slightly.
5. Put the walnuts on a baking sheet and toast in the oven for 8 minutes or until golden. Leave to cool slightly, then whizz in a food processor to fine crumbs (or finely chop). Toss with the cheese. Transfer the onions to the pastry case, flatten down slightly, then scatter over the walnut mix. Turn the oven down to 180°C/ fan160°C/gas 4. Bake the tart for 20 minutes or until golden brown. Leave the tart to cool to room temperature before slicing. Serve with a bitter leaf salad, if you like.