Place flour, brown sugar and salt in a large bowl and mix. Add cubed butter and using your fingertips rub it into the flour mix, until it resembles the texture of large breadcrumbs.
Add ice cold water and bring the dough together. Knead it few times, then shape into disc, wrap and refrigerate for 1-2 hours.
Prepare the frangipane by mixing soft butter with brown sugar until light and fluffy. Add egg and mix again. Add ground almonds and fold it all together to create a paste-like texture. Set aside.
Mix the sliced peaches with lemon juice, vanilla, Tate & Lyle pure cane caster sugar and cornflour. Set aside. Preheat the oven to 180 C (170 C fan).
On a large piece of baking paper, roll the chilled pastry into a large circle, roughly about 11' in size. Trim the edges (use a back of the large dinner plate or a large bowl to trace the circle shape) of the pastry. If pastry becomes too soft, place it back in the fridge for about 20 minutes. Spread the frangipane filling in the middle, leaving a small border around the edge. Arrange the sliced peaches on top. Fold the edges of the pastry to slightly overlap the fruit.
Brush with eggwash and sprinkle with demerara sugar. Place on a baking sheet and bake at 180 C (170 C fan) for 30- 35 minutes until golden brown.
Allow the galette to cool down slightly, then brush some peach or apricot jam on top of the fruit to create the glaze. Serve with some cream or a scoop of ice cream.
If the crust dough becomes too soft, place it back in the fridge for about 20 minutes before using it again
If making ahead, prepare the dough and frangipane filling and place both in the fridge for up to 2- 3 days. Both can also be frozen for up to 3 months. Thaw in the fridge overnight.