Remove your pastry block from the fridge or freezer according to packet recommendations before using.
If making your own pastry, place the flour into your food processor. Pulse a few times and then add the chilled butter.
Pulse until it resembles coarse crumbs.
With the motor running, add the water 1 tbsp at a time. The pastry should start to come together. Tip out onto a lightly floured surface. Form a thick disc, wrap in cling film and place into the fridge for 25 – 30 minutes before using; or up to two days. Remove from the fridge 15 – 20 minutes before rolling, depending on the temperature of your kitchen if you make the pastry a couple of days in advance.
Preheat your oven to 190C (375F).
Roll your pastry out to form a 12 – 14 inch round between two pieces of parchment paper. Transfer the pastry on the baking paper to a large baking tray.
Trim the edges of the pastry and then lightly brush the entire surface with the beaten egg.
Mix together the raspberry jam and cornstarch in a bowl. Add the raspberries and mix to combine the two.
Place raspberry mixture into the centre of the pastry leaving about 2 inches worth of pastry as a border.
Fold in the edges of the pastry over the filling. Brush right around the edges with the beaten egg, sprinkle with some sugar if desired and bake in the preheated oven for 15 – 18 minutes.
Serve warm with ice cream, or can be served chilled and lightly reheated if desired. Stored cooked galette in the fridge where the filling will firm up a bit more.