Preheat oven to 350°F (175°C). Line a 9x13-inch metal baking pan with parchment paper, leaving an overhang on the sides for easy lifting. In a medium bowl, stir together flour, powdered sugar, melted butter, vanilla, and salt until a soft dough forms. Press evenly into the bottom of the prepared pan. Bake for 15 minutes until lightly golden around the edges.

While crust bakes, toss the thinly sliced apples with brown sugar, flour (or cornstarch), cinnamon, nutmeg, and fresh lemon juice in a large bowl until every slice is glossy and well-coated.

In a separate bowl, combine oats, flour, brown sugar, and cinnamon. Add cold cubed butter and cut in with a pastry cutter or your fingertips until crumbly, pea-sized clusters form.

Arrange the sliced spiced apples evenly over the warm par-baked shortbread crust. Scatter the oat streusel crumble generously over the top. Return pan to the oven and bake for 30 to 35 minutes until the streusel is deeply golden brown and the apple filling is bubbling.

Cool completely in the pan for 1 hour at room temperature, then chill in the refrigerator for 1 hour so the shortbread sets firmly for razor-sharp square cuts. Lift out using the parchment overhang, slice into 16 bars, drizzle generously with warm salted caramel, and serve with vanilla bean ice cream!