First prepare the pastry. Place the flour and salt in a mixing bowl and whisk together. Chop the butter into cubes and rub into the flour using your fingertips. When it has a breadcrumb texture, mix in the granulated sugar, followed by the beaten egg and 1 tablespoon of cold water. Keep adding water, a tablespoon at a time, combining and squeezing the mixture with your hands, until the pastry comes together into a ball (2 tablespoons of water usually suffice). Wrap the ball in clingfilm and chill in the fridge for 30 minutes.
While the pastry is chilling, thoroughly grease and line the base of a 28cm loose-bottomed tart or quiche tin and preheat the oven to 200˚C/180˚C fan/gas mark 6.
Remove the pastry from the fridge and using a lightly floured rolling pin, roll into a circle on a sheet of lightly floured greaseproof paper on a smooth surface. Continue rolling to enlarge the circle, sprinkling the surface of the pastry with flour for ease of rolling, until it is 7.5cm wider than the tin on all sides.
Lifting up the pastry, greaseproof paper and all, and gently flip and push into position in the tin. Discard the greaseproof paper. Roll the rolling pin over the top edge of the tin firmly, then pull away the excess pastry hanging over the edges. Prick the base of the tart all over with a fork and place in the fridge.
Re-roll the leftover pastry out on a lightly floured surface and cut into 7–10 2cm-wide strips. Use a pastry or pasta wheel when doing this to create nice serrated edges. Place the strips in the fridge.
Peel and core the apples, then roughly grate into a bowl using a cheese grater. Sprinkle the cinnamon over the fruit and mix well. Remove the pastry-lined tin from the fridge and spoon the grated apple inside. Level the surface of the grated apple, then position the cut pastry strips over the top of the panade in a criss-cross pattern. Trim the strips using the pastry wheel if there is any overhang. Brush all the visible pastry with the milk, then sprinkle over the demerara sugar.
Place the tart in the oven and bake for 55 minutes, or until the pastry is cooked. Remove from the oven and gently lift out of the tart tin and set onto a wire rack to cool. Enjoy the panade hot or at room temperature with vanilla ice cream.