1. Preheat oven to 350°F/177°C.  Line an 8x8-inch square baking pan with parchment paper, allowing it to overhang on all sides. Set aside.
2. In a medium mixing bowl, combine apples, lemon juice, and granulated sugar.  Allow apples to sit while making the shortbread crust, stirring occasionally.

For the shortbread crust

1. Using a hand-held mixer or stand mixer fitted with the paddle attachment beat the butter and sugar together on medium speed until light and fluffy, about 2 minutes.  Add the vanilla and mix until combined.  Add the flour and salt and mix until combined and no longer crumbly (the dough will start to come together). Press the mixture evenly into the prepared baking pan. Bake for 15 minutes or until the edges are lightly golden.

For the apple filling

1. In a medium sauté pan, melt the butter over medium heat.  Add the flour and whisk for 2 minutes.  Add the liquid from the apple mixture, brown sugar, cinnamon, and nutmeg and mix well.  Bring to a boil and allow to simmer 2 minutes or until the juices are thick and a caramel-like consistency.  Toss in the apples and continue to sauté for 2 minutes.  Remove from the heat.

For the streusel topping

1. In a medium bowl, combine the flour, walnuts, sugar, cinnamon, and salt. Using a silicone spatula, stir in the melted butter until the mixture is thick and crumbly.
2. Layer the apples tightly and evenly over the shortbread crust to avoid any gaps between the apples.
3. Sprinkle the streusel over the apple layer and bake for 30–35 minutes or until the streusel is golden brown.
4. Remove the pan from the oven and allow to cool 30 minutes in the hot pan on a wire rack.  Transfer the pan to the refrigerator to chill for at least 2 hours.
5. Cut into 9 bars and serve cold, at room temperature, or slightly warmed.  If desired, drizzle with caramel sauce just before serving.