We like a mix of tart and sweet apples; you can also use Empires or Cortlands (tart) and Fuji, Jonagolds, or Braeburns (sweet). If using an enameled skillet, we recommend placing a baking sheet underneath while baking to catch any juices that might bubble over.
1. Heat 10-inch cast-iron skillet over medium heat for 3 minutes. Melt 1 tablespoon butter in skillet. Add half of apples, cover, and cook until apples begin to release their juice, about 4 minutes. Uncover and continue to cook, stirring occasionally, until apples are tender and golden brown, about 5 minutes. Transfer apples and their juice to rimmed baking sheet. Repeat with remaining 1 tablespoon butter and remaining apples; transfer to sheet. Spread apples into even layer and let cool to room temperature, about 30 minutes.
2. Adjust oven rack to lowest position and heat oven to 400 degrees. Grease clean, dry, cooled skillet. Loosely roll 1 crust around rolling pin and gently unroll it onto prepared skillet. Ease crust into skillet by gently lifting and supporting edge of dough with your hand while pressing into skillet bottom and corners with your other hand. Leave any overhanging dough in place.
3. Whisk cider, maple syrup, cornstarch, salt, lemon zest and juice, and cinnamon together in large bowl until smooth. Add cooled apples and any accumulated juices and toss to combine. Transfer apple mixture to dough-lined skillet, mounding apples slightly in middle. Loosely roll remaining crust around rolling pin and gently unroll it onto filling.
4. Trim any overhanging dough to 1/2 inch beyond lip of skillet, then ease edge of top crust into skillet until flush with bottom crust. Gently press top and bottom crusts together to seal. Roll in edge of crust, then press rolled edge against sides of skillet using index finger to create attractive fluted rim. Using paring knife, cut eight 2-inch vents in top crust in circular pattern. Brush crust liberally with egg wash and sprinkle with sugar.
5. Transfer skillet to oven and bake until crust is deep golden brown and filling is bubbling, 40 to 50 minutes, rotating skillet halfway through baking. Transfer skillet to wire rack and let pie cool until filling is set, about 2 hours. Serve slightly warm or at room temperature.