Combine cream, 1/2 cup brown sugar, 3 tablespoons butter, and 3 tablespoons white sugar in a small, heavy saucepan over medium-low heat. Cook, stirring constantly, until sugars dissolve, 2 to 3 minutes. Bring sauce to a simmer, stirring frequently. Cook until slightly thickened, about 7 minutes. Remove from heat and stir in whiskey.
Combine 1 3/4 cups flour, 1/3 cup white sugar, 2 teaspoons orange zest, and salt in a food processor; pulse to briefly combine. Scatter 3/4 cup butter pieces over the flour mixture and process until dough starts clumping together.
Press dough over the bottom of a 9x13-inch baking pan. Brush egg white over dough.
Bake in the preheated oven until lightly golden, about 18 minutes. Cool on a wire rack.
Combine apples, apple juice, 1/4 cup brown sugar, 1/4 cup white sugar, lemon juice, and apple pie spice in a large skillet. Bring to a boil; cook, stirring frequently, until mixture thickens and looks dry, 18 to 20 minutes. Remove from heat and let cool. Stir in 1 tablespoon orange zest.
Place 1 1/4 cups flour and 1/3 cup white sugar in a food processor; pulse briefly to combine. Scatter 1/2 cup butter pieces and walnuts over flour mixture. Process until topping mixture starts to clump together. Pour into a bowl. Squeeze topping mixture with your fingertips to form clumps the size of peas.
Spread cooled filling evenly over dough. Drizzle 1/2 the caramel sauce on top. Sprinkle topping evenly over caramel.
Bake in the preheated oven until edges are light golden, about 35 minutes. Place pan on a wire rack to cool for 10 minutes. Drizzle remaining caramel sauce on top. Let cool completely, about 15 minutes. Cover with aluminum foil and refrigerate until firm, about 1 hour. Cut into bars.
You can make the caramel sauce ahead and refrigerate it for up to 2 days. Reheat in microwave on medium power until slightly warm. You can make the base ahead and let it stand at room temperature, 8 hours to overnight. You can make the filling, cover it tightly with plastic wrap, and refrigerate it, 8 hours to overnight. Bring to room temperature before using.
The whole pan can be stored, tightly covered, in the refrigerator for up to 2 days or frozen for up to 2 weeks. Cut into bars before serving.
Substitute brandy for the whiskey if preferred. Substitute cinnamon for the apple pie spice if preferred. Replace orange zest with lemon zest if preferred. Add 1 tablespoon orange zest to the filling if desired.
Per Serving: 134 calories;6.9 g fat;
17.6 g carbohydrates;
1.2 g protein;
16 mg cholesterol;
66 mg sodium.