Make the crust by sifting together the flour, coconut sugar, and salt. Add in the cubed butter and cut it into the dry ingredients with your hands until the mixture resembles coarse breadcrumbs.
Drizzle in the water and bring it together with a fork until it forms a loose ball.
Dump the pastry onto a lightly floured surface and shape the dough into a disc. Do not overwork it. Cover entirely with parchment paper and refrigerate for 30 minutes.
Meanwhile, get on with the filling by folding together the strawberries, coconut sugar, arrowroot powder, vanilla extract, sea salt, and chopped chocolate. Keep mixing until all of the arrowroot powder disappears, then set aside.
Preheat the oven to 190°C (375°F) and line a baking tray with a silicone mat or non-stick paper.
Lightly flour a work surface and roll the chilled dough into a 30cm (12 inch) diameter circle. Lay the pastry dough on the prepared tray and brush egg white over the surface.
Spoon the strawberry filling into the centre of the dough, leaving 4cm (2 inch) around the edge bare.
Gently fold the edge up and over the filling, taking care to crimp together the folds as you go.
Brush the top of the pastry with the egg yolk and evenly sprinkle over the coarse sugar. Bake the galette for 40-45 minutes, rotating the pan at the halfway mark.
Remove the galette from the oven and allow to cool on the tray for 15-20 minutes before serving to let the juices firm up a bit. If desired, sprinkle over some chopped fresh mint and a dusting of icing sugar.