To make the galette dough, mix the dry ingredients (flour, icing sugar, ground almonds and a pinch of salt) in a medium bowl. Rub in the butter then make a well in the centre. Crack in the egg with 2 tbsp ice-cold water and use a dinner knife to bring it together into a rough galette dough.
Wrap the galette dough in clingfilm (plastic wrap) and chill in the fridge for 30 minutes. Preheat the oven to 190°C/Fan 170°C/ Gas 5.
Slice the peaches and place in a bowl with the raspberries. Add a squeeze of lemon juice to the fruit, toss, then sprinkle over 2tbsps granulated sugar, 1 tbsp flour, the cinnamon and nutmeg, then mix. Roll out the chilled dough to a rough 30cm circle, 3mm thick. Transfer to a baking sheet lined with greaseproof paper.
In a small bowl mix the jam with a little squeeze of lemon juice. Spread most of the jam over the base of the pastry leaving a 2-inch border. Arrange the fruit over the jam. Fold the pastry border over fruit then brush the egg wash over the border.
Brush the fruit with the remaining jam and sprinkle all over with the remaining caster sugar. Bake the galette for 30 minutes until the pastry is golden brown. Serve hot with vanilla ice cream.