Preheat and prep pan: Preheat the oven to 350°F and line an 8x8-inch baking pan with parchment paper, leaving overhang on two sides for easy removal later.

Mix crust ingredients: In a bowl, combine the flour, granulated sugar, and salt, then cut in the cold butter with a pastry cutter or your fingers until the mixture looks like coarse crumbs.

Press and bake crust: Press the crumbly mixture firmly into the bottom of the prepared pan and bake for 12 minutes, until the edges turn light golden.

Toss the apple filling: While the crust bakes, toss the diced apples with lemon juice, brown sugar, cinnamon, and the tablespoon of flour until every piece is coated.

Spread filling over crust: Once the crust comes out of the oven, spread the apple mixture evenly over the warm base.

Make the crumble topping: Stir together the oats, chopped pecans, and melted butter until the mixture clumps together in small pieces.

Add topping and bake again: Scatter the crumble over the apples and return the pan to the oven for 16 to 18 minutes, until the apples are tender and the topping turns golden brown.

Cool completely: Let the pan cool on a wire rack for about 30 minutes so the layers set before slicing.

Drizzle with caramel: Warm the caramel sauce slightly if needed, then drizzle it evenly over the cooled bars.

Chill to set: Place the pan in the fridge for 20 to 30 minutes so the caramel firms up and the layers hold together when sliced.

Slice into bites: Lift the bars out using the parchment overhang and cut into 24 small squares with a sharp knife.

Finish and serve: Sprinkle with flaky sea salt if you like, then serve the bites at room temperature or slightly chilled.