Begin by making the crust: in a large bowl, mix together the flour and salt, then drop in the butter cubes and work it in with your finger tips and/or rubbing it into the flour with your palms. Work quickly to not melt/warm the butter too much. When your mixture is coarse, drop in a few tablespoons water, work it in with a fork (or your fingers), and continue adding water til the dough comes together when pressed. Give the dough a quick knead to gather all the dry bits, flatten into a disk and wrap with cling wrap. Refrigerate for about an hour.
Preheat the oven to 400°F. Roll out the chilled dough to ⅛ inch between two large pieces of parchment. Transfer the parchment to your baking sheet and chill.
Meanwhile, peel core and slice apples to ⅛ inch. Toss with 2 tablespoon granulated sugar. Set aside.
Take the rolled dough out of the fridge, and remove the top sheet of parchment. Brush the apple jelly over the crust leaving 1–1.5 inches of crust bare.
Top the jelly with the sliced apples arranging them in concentric circles to make it pretty and still keeping the 1–1.5-inch edge bare.
Gently fold over the bare crust over towards the middle to make a rustic edge. With your palms, press and push the fold towards the middle a little to “tighten” the galette.
Brush the crust with milk and sprinkle with the crust coarse sugar. Sprinkle a little more granulated sugar over the apples.
Bake for 20 minutes then lower the oven to 375°F and bake for another 30 minutes until the crust is golden brown delicious and the apples are soft.
Let cool about 20 minutes before serving