Pre-heat the oven to 180°C/350F/gas mark 4. Make the pastry in a large bowl by tossing the diced butter in the flour, sage, parmesan and a pinch of salt. Add just enough ice-cold water, about 6-8 tablespoons, to bring it together into a firm dough that is not too sticky. There should be no loose pieces of fat or flour left in the bowl.
Turn onto a floured work surface and roll into a rectangle of about 1 cm thick. Fold the top third into towards you into the middle, and the bottom third up over it - just like folding a letter. Turn the pastry by 90°, roll out to a rectangle again and repeat the folding. Repeat this turning, rolling and folding process 4 or 5 times, keeping the board and rolling pin lightly floured. Wrap the pastry in cling film and chill for at least an hour before using.
Whilst the pastry is chilling, make the filling. Cut each Fenland celery bunch into half through the root, then each half into quarters leaving you with 8 pieces. Melt the butter with the oil in a frying pan that can go in the oven. Add the sugar, balsamic vinegar and celery and season with a little salt and freshly ground black pepper. Cover the mix loosely with a piece of damp greaseproof paper and gently sweat for 15-20 minutes until the celery is soft & tender when pierced with the point of a knife.
Remove the greaseproof paper, turn up the heat and fry for a further 5 minutes until lightly caramelised. Turn off the heat and spread the celery out evenly in a single layer over the base of the pan. Allow cooling a little.
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