To make the pie:
Have all of your ingredients ready and chilled.
Combine the flour, salt and sugar in a food processor. Add the butter and lard.
Use the food processor to work the butter and lard into the flour. Work the fats into the flour until it starts to stick together when squeezed, but you want to keep some of the fat in pea-size pieces. If it is too soft and it won't stay in pieces, place the bowl in the freezer for about 15 minutes and then try again.
Whisk the egg, add it along with the water and vinegar to the flour/lard mixture.
Stir this with your hand or a wooden spoon, but be very gentle. The goal is to keep the pea-size chunks of fat intact. It will start to look shaggy and not dry. It should hold together when you squeeze it in your hand.
Gently knead the dough into a ball. Divide the dough into two pieces and create round disks. Wrap the dough in plastic and refrigerate for at least an hour, or overnight.
Make the filling:
Toss together the cut apples, sugar, salt, cinnamon and zest in a bowl.
To make the galette:
Preheat oven to 400 degrees F. Line a cookie sheet with parchment.
On a floured surface, roll out the chilled pie dough. It should be a 1/8-inch-thick circle. Dust off any extra flour and lay it on the parchment.
Pile the apples in the middle of the dough.
Fold the dough up, so it covers at least 2 inches of the filling, more is ok. Brush the edge with egg wash and dust generously with sugar.
Loosely tent the top of the galette with foil. (Most apple pies will have a top crust that traps the steam and helps to cook the apples. Since this is an open top, we need to create that closed environment with the foil.)
Bake for 30 minutes, then turn the oven down to 350 degrees F and bake for an additional 30 minutes. Remove the foil and bake for about 10 more minutes.
Serve warm or at room temperature.