Preheat oven to 350°F (175°C). Line an 8×8‑inch metal baking pan with parchment. Stir 1 ½ cups flour, ½ cup sugar, and salt. Add melted butter and mix until crumbly. Press into pan. Bake 15 minutes until edges are pale gold.

Toss apples, ⅓ cup sugar, cornstarch, cinnamon, nutmeg, and lemon juice in a saucepan. Cook over medium heat, stirring, about 5 minutes until juices thicken and apples are just tender. Cool 5 minutes.

Combine oats, ¼ cup flour, and brown sugar. Cut in cold butter until mixture forms coarse crumbs.

Spread apple filling over crust, then sprinkle topping evenly. Bake 30‑35 minutes until topping is deep golden and filling bubbles at edges. Cool completely in pan on a rack.

Lift bars out with parchment, drizzle with caramel sauce, and sprinkle with flaky salt. Slice into 9 squares and serve.