200g mixed blackberries, blueberries and raspberries
vanilla ice cream, to serve (optional)
Preheat the oven to 200°C, gas mark 6. Roast the hazelnuts on a baking tray for 8-10 minutes. Leave to cool, then blitz 50g to coarse crumbs in a food processor. Roughly chop the remaining nuts and set aside.
Line a large baking tray with baking paper. Put 190g flour and a pinch of salt in a bowl. Rub in 170g butter until the mix resembles breadcrumbs. Stir in 80ml cold water and bring together to form a dough.
Turn out onto a lightly floured surface. Roll into a rough 25cm circle and transfer to the prepared tray. Chill for 30 minutes.
Beat the remaining butter and 50g sugar until fluffy. Whisk in the unbeaten egg, then fold in the blitzed hazelnuts and remaining flour. Spread the frangipane over the chilled galette base, leaving a 4cm border.
Toss the plums and berries with the remaining 2 tbsp sugar, then arrange on top of the frangipane. Fold the pastry border partially over the filling, crimping as you go. Brush with the beaten egg and bake for 25-30 minutes, or until the frangipane has just set in the centre and the pastry is crisp and golden brown.
Scatter over the chopped hazelnuts. Serve the galette with vanilla ice cream, if you like.