Add flour, sugar, and salt to a large bowl.
Use a coarse grater to grate cold butter in the bowl with flour.
Use a stand mixer with kneading attachment to mix the butter in the flour. Mix only for a minute so the butter doesn't overheat. Then add cold milk and mix until you don't see any more dry flour. The dough will still look a little dry but if it forms a ball when squeezed, it's done. Wrap it in plastic wrap and place in the fridge for 30 minutes to set and cold down.
Meanwhile cut apples into thin slices and sprinkle with cinnamon.
Preheat the oven to 180 °C. Line a baking tray with baking paper.
When the dough is chilled roll it out to about 3-4 mm thick into a round shape. Spread the jam on the dough but leave 2-3 cm edge clean. Place apples over the jam. Again leave 2-3 cm crust edge and fold it over the apples to form a galette.
Sprinkle some more unrefined sugar over the galette.
Bake for 30 minutes or until the crust is golden brown.
Leave to cool until moving from the baking tray