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, combine flour, powdered sugar, melted butter, and salt. Stir with a fork until a soft dough forms. Press firmly and evenly across the bottom of the prepared pan. Bake for 15 minutes until lightly golden around the edges. Set aside to cool slightly while preparing fillings.

In a large bowl, beat softened cream cheese and sugar with an electric hand mixer for 2 minutes on medium speed until completely smooth and lump-free. Add eggs one at a time and vanilla extract, beating on low speed just until incorporated. Pour cheesecake batter over the warm crust and spread smooth.

In a separate bowl, toss finely diced apples with sugar, cinnamon, and nutmeg until every apple piece is spiced. Spoon the apples evenly across the cheesecake layer.

In a small bowl, whisk rolled oats, flour, and brown sugar. Cut in cold cubed butter using a pastry cutter or your fingertips until crumbly, pea-sized streusel clusters form. Scatter streusel evenly over the apple layer.

Bake for 32–36 minutes until the streusel is golden brown and the cheesecake is set around the edges with a very slight center jiggle. Cool completely at room temperature for 1 hour, then refrigerate for at least 3 hours (or overnight) to set firm.

Use the parchment paper overhang to lift the bars out onto a cutting board. Cut into 16 clean squares with a hot chef knife. Drizzle generously with warm salted caramel and sprinkle flaky sea salt just before serving.