To make the pastry, combine the flour and salt in a medium bowl and grate or cut in the butter, blending it with a fork, pastry blender or rubbing it with your fingers until combined and crumbly, with some bigger pieces about the size of a pea. Add the water and stir just until the pastry comes together. Gather into a ball, shape into a disc and let rest or refrigerate for at least half an hour, or up to a few days in the fridge.
Set a large, heavy skillet over medium-high heat, add the butter and sauté the apples for 4-5 minutes, until softened and turning golden. Sprinkle with the sugar, cinnamon and salt and stir until the sugar starts to melt with the heat and juice of the apples. (It shouldn't be too runny though - if it seems to be, keep cooking it for a few more minutes.)
Preheat the oven to 400F. Divide the pastry into 4-6 pieces and on a lightly floured surface, roll each piece out into a 5-7 inch circle. Put a spoonful of filling in the middle of each, fold over to cover and then seal by folding the edge up from underneath, to contain any leaks. Crimp if you like, or just haphazardly pinch - they'll look and taste great either way. It's easiest to assemble them on a parchment-lined sheet, otherwise just move them over onto one with a spatula when they're done. Brush with a little beaten egg and cut a couple holes in the top to allow steam to escape.
Bake for 20 minutes, or until nice and golden. Makes 4-6.