For the dough:
1 3/4 cups all-purpose flour
2 tablespoons granulated sugar
1 teaspoon apple cider vinegar
Pinch of kosher salt
1 stick cold unsalted butter, diced
1 large egg
Filling & Topping
4 to 5 large apples, peeled and sliced into 1/4 inch slices (about 4–5 cups slices)
1/2cup (50g) packed light or dark brown sugar
1/4 cup white sugar
1 and 1/2 Tablespoons (12g) all-purpose flour
2 teaspoons fresh lemon juice (to prevent apple browning)
1 teaspoon ground cinnamon
1/4 teaspoon ground nutmeg
egg wash: 1 large egg beaten with 1 Tablespoon milk
optional: 1/3 cup (40g) chopped walnuts
Make the dough: Pulse the flour, granulated sugar, vinegar and salt in a food processor until combined. Add the butter and pulse until the mixture looks like coarse meal with pea-size pieces of butter. Whisk the egg with 2 tablespoons water; add to the food processor and pulse until a dough just starts to form. Turn out onto a piece of plastic wrap; shape into a disk, wrap tightly and refrigerate until firm, at least 1 hour.
As the dough chills, prepare the filling: Mix the apples, brown sugar, flour, lemon juice, cinnamon, and nutmeg together in a large bowl. Cover tightly and let sit until the dough is ready.
Preheat oven to 375°F (190°C). Line a large baking sheet with parchment paper or a silicone baking mat. Set aside.
Roll out the dough into a 12-inch round between 2 sheets of floured parchment paper. Remove the top piece of parchment and invert the dough onto the prepared baking sheet; remove the other piece of parchment.
Arrange the apples (not the juices) into the center of the dough, leaving a 2-3 inch border all around. Gently fold the edges of the dough over the apples, overlapping the dough as necessary. Press gently to seal the edges. Drizzle 3 Tablespoons of salted caramel over the filling (not the crust). Brush the crust edges with egg wash and sprinkle the whole thing with walnuts, if using.
Bake until the filling is bubbly and the crust is golden brown, about 35-38 minutes. Allow to cool on the baking sheet for 10 minutes before slicing and serving.
Serve with extra salted caramel sauce drizzled on top.