If your oil is at room temperature, refrigerate for 30 mins before using.
Mix the flour, sugar and salt in a bowl. Add the cold vegetable oil and mix until a crumbly dough forms.
Add 1 tbsp water at a time and mix the dough with your hand until dough comes together and you have a nice smooth dough. Freeze for 10 mins while you work on the filling.
Mix all filling ingredients in a bowl. When your dough has chilled, roll it between 2 parchment papers to the desired thickness (mine was about 1 cm). Take the parchment paper on which the dough is and place in a pan. Fill it with the peaches mixture in the center then fold in the edges to create a galette (look at the photos).
Optional: mix the egg with a splash of milk and brush all galette edges.
Bake in preheated oven (180°C) for 20-25 mins or until golden brown on top.