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To make the pastry, sieve the flour into a bowl and stir in a pinch of salt and the sugar. Rub the butter into the flour using your fingertips until the mixture resembles coarse breadcrumbs. Using a cutlery knife, stir in just enough cold water to bring the pastry together into a dough (you can use up to 5-6 tbsp). Form the dough into a ball and flatten into a disc. Wrap and chill in the fridge for 30 mins.
For the filling, cook the onions with the oil and thyme leaves in a frying pan over a medium heat for 20 mins until the onions have softened and are translucent but not coloured. Season, remove from the heat and leave to cool.
Heat the oven to 200C/180C fan/gas 6. Roll the chilled pastry out on a lightly floured surface into a large rectangle, around 3mm thick. Transfer the pastry rectangle to a large baking tray, spoon the onion filling into the centre and spread it out evenly, leaving a 5cm border around the edges. Sprinkle over the tarragon, then arrange the courgette slices over the onions in overlapping rows. Season the courgettes and sprinkle with some extra thyme.
Fold the sides of the pastry over so they’re overlapping the edge of the filling, leaving the middle exposed. Gently press the pastry folds down at the corners to secure them, then brush the pastry with the beaten egg. Bake the galette for 40-50 mins until the pastry is golden brown and the courgettes are soft and slightly golden. Leave to cool and firm up for a few minutes before serving. Will keep chilled for up to three days. Leave to cool first.