Preheat oven to 350°. Stir together first 5 ingredients in a large bowl. Let stand 30 minutes, stirring occasionally. Stir in flour.
Unroll and stack piecrusts on parchment paper. Roll into a 12-inch circle. Mound apple mixture in center of piecrust using a slotted spoon (mixture will be slightly runny), leaving a 2- to 2 1/2-inch border. Fold piecrust border up and over fruit, pleating as you go, leaving an opening 5 inches wide in center. Drizzle apples with any remaining juice in bowl. Brush piecrust with egg, and sprinkle with 1 Tbsp. sugar. Slide parchment paper onto a baking sheet.
Bake at 350° for 50 to 55 minutes or until filling is bubbly and crust is golden brown. Cool on a baking sheet on a wire rack 30 minutes. Serve warm with Buttermilk Whipped Cream.