Preheat oven to 350°F (175°C). Line a 9x9 inch square baking pan with parchment paper, leaving an overhang on the sides for easy lifting.

In a medium bowl, mix crushed graham cracker crumbs, melted butter, brown sugar, and cinnamon until evenly moistened. Press firmly into an even layer in the bottom of the prepared pan. Bake for 10 minutes, then remove and cool slightly.

In a large bowl using an electric hand mixer, beat softened cream cheese and granulated sugar on medium speed until smooth and lump-free (about 2 minutes). Beat in eggs one at a time, followed by sour cream and vanilla extract, mixing just until combined. Pour over the warm baked crust and smooth evenly with an offset spatula.

In a separate bowl, toss diced apples with brown sugar, cinnamon, nutmeg, and lemon juice. Scatter the spiced apples evenly across the top of the cheesecake layer.

In a small bowl, combine rolled oats, flour, and brown sugar. Cut in cold cubed butter with a pastry cutter or your fingertips until coarse, crumbly clusters form. Sprinkle the streusel evenly over the apples.

Bake for 32 to 35 minutes until the cheesecake center is set and the streusel topping is golden brown. Remove from oven and cool to room temperature on a wire rack for 1 hour, then refrigerate for at least 3 hours (or overnight) until thoroughly chilled.

Lift bars out using parchment handles and slice into 16 neat squares with a warm sharp knife. Drizzle generously with warm salted caramel sauce, sprinkle with flaky sea salt, and serve!