1. To make the crust: mix together the flour, sugar and salt in a bowl. Cube the butter and add to the dry ingredients. Rub the mixture between your fingers till it has formed a soft crumble. Drizzle in 5 tablespoons of water, stirring gently with your hand till it starts to form a dough. Continue to knead in the bowl for about 2mins until all the dough comes away from the bowl and it becomes smooth. Pat the dough into a disk, wrap with cling wrap, and refrigerate for 30 minutes till cool.
2. Preheat the oven to 180°C. Line a baking tray with paper and set aside.
3. In a small pot on the stove, add all the ingredients for the caramel sauce. Using a whisk, bring the mixture to the boil over a medium to high heat and stir till it has combined into a smooth sauce. Simmer the sauce for 15mins till thick and glossy and remove from the heat.
4. For the filling; slice the apples in half, core the apples and slice downward into semi-circle slices roughly half a cm thick. Add them to a bowl with the cinnamon, nutmeg, sugar, melted butter, lemon juice and zest. Make sure the apples are fully coated; mix the milk and flour into a paste before adding to the apples and stirring.
5. After about 20-30mins the dough should be cold to touch, take it out of the wrap; knead lightly for 20 seconds to soften the dough and roll out in between 2 pieces of baking paper to a large circle about 30cm in diameter.
6. Starting in the centre of the dough, layer your apples overlapping in a circular spiral motion and stop when you get to approx. 2 inches from the edge of the dough. Start to fold the dough edges up over the apple, where the dough overlaps, carefully fold it and press to keep the shape.
7. Create an egg wash for the crust by lightly beating the egg and tablespoon of milk, brush lightly over the crust and sprinkle the demerara sugar over the entire galette and place in the oven for 30mins.
To serve; remove the apple galette from the oven, generously drizzle the salted caramel sauce, crush and sprinkle the macadamias, and heap a big spoon of double thick cream in the centre.
If the salted caramel sauce becomes too firm, add an additional tablespoon of cream and return to a medium heat. Using a whisk, remix the sauce and it will return to a soft sticky sauce.
You can also serve this galette with your favourite ice cream or yogurt.