To make the pastry, sift the flour into a large bowl. Cut the butter into cubes and combine with the flour and a pinch of salt. Work to a breadcrumb texture. Make a well in the middle and pour in the water. Work the mixture to a paste, either in the bowl or transfer to a well-floured work surface. Cover with cling film and allow to rest for 30 minutes.
Preheat the oven to 200°C/180°C fan/Gas 6. Roll out the pastry on a lightly floured surface and cut to line a 28cm (11in) tin. Prick the base and chill for 30 minutes. Bake blind using baking beans for 15-20 minutes until the pastry is cooked, trim off any excess.
Cook the rice according to instructions. Drain and allow to cool. Heat the olive oil and butter in a large frying pan and gently sweat the leeks and onions for about 10-15 minutes until soft. Add the mushrooms and rosemary and fry for a further 6-7 minutes, then stir in the chestnuts, cooked rice and most of the sage and cook for a final 2 minutes. Season to taste.
Spoon the filling into the pastry case and return to the oven for 10-15 minutes before serving to warm through. Fry the remaining sage leaves in olive oil, dry on kitchen paper then use to garnish the tart. Drizzle with balsamic vinegar and serve with a green salad.
Use any leftover ingredients to make either individual small tarts or large rice bake with other vegetables, nuts or even dried fruit.
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