Combine the flour, sugar, and salt in a large mixing bowl and stir briefly.

Add the cold butter cubes and use your fingertips to rub the butter into the flour until the mixture looks like rough, uneven crumbs with some pea-sized butter pieces still visible.

Add ice water one tablespoon at a time, stirring with a fork after each addition, until the dough just comes together when you press a handful - it should not be sticky.

Turn the dough onto a lightly floured surface, press it into a flat disk, wrap tightly in plastic wrap, and refrigerate for at least 30 minutes.

Pat the peach slices dry with paper towels, then place them in a bowl.

Add the honey, sugar, cornstarch, thyme leaves, vanilla extract, and lemon juice. Toss gently until the peaches are coated. Set aside for 5 minutes.

Heat the oven to 200 C / 390 F. Line a large baking sheet with parchment paper.

On a lightly floured surface, divide the dough into 6 equal pieces. Roll each piece into a rough circle about 14 cm / 5.5 inches wide and 3 mm thick.

Transfer the dough rounds to the prepared baking sheet, spacing them 2 cm apart.

Arrange the peach slices in overlapping layers in the center of each dough round, leaving a 4 cm border all the way around. Drizzle any remaining filling juices from the bowl over the fruit.

Fold the dough edges up over the outer peach slices, pleating as you go to form a rough border. Press the folds lightly so they hold together.

Brush the folded crust edges with egg wash. Sprinkle coarse sugar generously over the crust and a little over the fruit.

Refrigerate the assembled galettes on the sheet pan for 15 minutes before baking.

Bake at 200 C / 390 F for 22 to 25 minutes, rotating the pan once halfway through, until the crust edges are deep golden brown and the peach filling is bubbling.

Remove from the oven and let the galettes cool on the pan for 10 minutes before serving - the filling sets a little as it cools.