In a large bowl, whisk together 2.5 cups flour, 1 tsp sugar, and 1/2 tsp salt. Add the 12 tbs (1.5 sticks) cold cubed butter and, using a pastry cutter or two forks, cut the butter into the flour until mixture resembles coarse peas with some pea-sized butter bits remaining.

Add liquids and chill

Stir together 6 tbs ice water and 1 tsp apple cider vinegar. Add 4–6 tablespoons of the ice water mixture to the flour-butter mixture, tossing gently after each addition, until dough just holds together when squeezed (you may not need all the water). Divide dough into two discs, flatten slightly, wrap tightly in plastic wrap, and chill at least 1 hour (or up to 2 days).

Roll

On a lightly floured surface, roll one disc into a 12-inch circle about 1/8" thick for the bottom crust. Fit into a 9-inch (2.5-inch deep) pie dish, allowing a 1-inch overhang. Roll second disc into a 10–11 inch circle for the top crust or cut into lattice strips if desired. Refrigerate while preparing filling.

Apple filling

Prepare apples

Combine 4 cups sliced Granny Smith and 2 cups sliced Honeycrisp/Fuji apples in a large bowl. Sprinkle 1/2 cup granulated sugar and 1/4 cup packed light brown sugar over apples and toss to coat.

Add thickener and flavor

In a small bowl, whisk 2 tsp cornstarch with a tablespoon of the apple mixture juices (or a bit of cold water) until smooth, then add to the apples. Add 1 tsp cinnamon, 1/4 tsp nutmeg, 1/4 tsp salt, and 1 tbsp lemon juice. Toss gently to combine. Let sit 10 minutes so juices release and cornstarch begins to thicken.

Cook briefly with butter (optional)

In a skillet over medium heat, melt 2 tbs butter and add the apple mixture. Cook 3–4 minutes, stirring, just until apples begin to soften and juices thicken slightly. This step reduces excess moisture and concentrates flavor. Remove from heat and let cool 10 minutes.

Salted caramel sauce

Make caramel

In a medium heavy-bottomed saucepan, combine 1 cup granulated sugar and 2 tbs water. Cook over medium heat without stirring (you can swirl the pan occasionally) until sugar dissolves and turns a deep amber color (about 8–10 minutes). Watch closely to avoid burning.

Finish caramel

Remove pan from heat and carefully add 6 tbs butter, whisking until melted. Slowly whisk in 1/2 cup warm heavy cream until smooth (mixture will bubble vigorously). Stir in 1/2 tsp salt and 1 tsp vanilla extract. Let cool slightly (5–10 minutes) until warm but pourable.

Assemble and bake

Preheat and prepare crust

Preheat oven to 400°F (205°C). Remove bottom crust-lined pie dish from fridge. Sprinkle a thin layer of breadcrumbs or 1 tbsp cornstarch (optional) on the bottom to absorb excess juices. Spoon the cooled apple filling into the crust, mounding slightly in center.

Add caramel

Reserve 1/3 cup of the warm caramel sauce. Pour the remaining caramel evenly over the apples, gently tossing with a spatula so apples are coated but not drowned.

Top crust and egg wash

Drape the top crust over the apples (or weave lattice). Trim overhang to 1/2 inch and fold under the bottom crust edge, crimping to seal. In a small bowl, whisk 1 egg with 1 tbsp milk to make egg wash. Brush the crust with egg wash and sprinkle with 1 tbsp coarse sugar if using. Cut vents in top crust if using a full top.

Bake

Bake at 400°F (205°C) for 20 minutes, then reduce oven temperature to 375°F (190°C) and continue baking 30–35 minutes more until crust is golden and filling bubbles through vents. If crust browns too quickly, tent with foil after 30 minutes.

Cool and finish with reserved caramel

Remove pie from oven and place on a wire rack. Allow to cool at least 2 hours (the filling will set). Warm the reserved 1/3 cup caramel slightly if thick; drizzle over the cooled pie and sprinkle an extra pinch of sea salt if desired. Let sit 15 minutes before slicing.