Maple Apple Galettes with Walnut Crumble

Steps

1. Make the crust first: whisk 300 g of the flour with 20 g of the granulated sugar and 4 g of the salt. Cut in 225 g of the cold butter until the pieces are pea-sized.
2. Stir the cider vinegar into the ice-cold water. Drizzle it over the flour mixture, tossing with a fork until the dough just holds together.
3. Divide the dough in half, flatten each half into a disk, wrap and chill for 1 hour.
4. Meanwhile make the crumble: rub together the remaining 40 g flour, the remaining 30 g butter, the brown sugar, oats, walnuts and the remaining 1 g salt until clumpy. Chill.
5. Make the filling: toss the apple with 60 g of the maple syrup, the lemon juice, cornstarch, cardamom and cinnamon.
6. Preheat the oven to 400°F (200°C) and line two baking sheets with parchment.
7. Roll each dough disk into an 11-inch round and set one on each sheet. Fan half the apple filling over each, leaving a 2-inch border, and fold the border over the fruit, pleating as you go.
8. Scatter half the crumble over each galette's exposed fruit. Brush the crust edges with the beaten egg and sprinkle them with the remaining 15 g granulated sugar.
9. Bake 40 to 45 minutes, until the crust is deep golden and the juices bubble. Brush the remaining 20 g maple syrup over the apples while hot.
10. Cool on the sheets for 30 minutes so the juices set, then cut each galette into 6 slices.