To make the shortcrust pastry, cut the cold (almost frozen) butter into the flour in a food processor and whiz till the mixture resembles breadcrumbs. Alternatively, rub the cold butter quickly into the flour using your fingertips till it resembles moist breadcrumbs.
Add 3 tbsp cold water into the processor and pulse for 20 seconds. Remove into a bowl and bring the dough together into a rough ball. Similarly, use a fork to stir in the cold water and use your hands to bind the dough, without kneading or too much handling. Wrap the pastry in clingfilm and chill in the fridge for 20 mins.
Gently heat 3 tbsp of butter in a pan till it foams and spatters. Heat it a little more till it turns brown and the dark particles sink to the bottom of the pan. Cool and whisk in the vanilla extract.
Meanwhile, core the apples and slice off the top and bottom part of each apple to get a flat surface (keep the skin on). With the mandolin on setting number 2, slice the apples into thin discs.
Preheat the oven to 232°C and place a pizza stone or a cookie sheet on the rack till it gets hot.
Roll out the chilled pastry on a parchment to a diameter of 12 inches. Arrange the apple slices in a circular overlapping pattern starting 2 inches from the edge of the pastry and finishing in the centre.
Brush the apple slices generously with the brown butter and fold the outer edge of the pastry to cover half the outer layer of apples. Press to seal any holes.
In a bowl, whisk the egg with the heavy cream. Brush the egg wash over all the exposed pastry and sprinkle the turbinado sugar all over the galette.
Carefully, remove the heated pizza stone / cookie sheet from the oven and gently slide the parchment with the pastry onto it.
Bake for 20 min until the tart is cooked and the pastry is browned.
Remove the cookie sheet and slide the parchment out. Brush with the warmed blueberry preserve and bake for a further 2 min.
Once done, the galette will slide smoothly onto a plate. Serve warm or cold.
Adapted from Brown Butter Apple Tart by Phyllis Grant on Food 52