01 - Preheat oven to 350°F. Line a 9x9-inch baking pan with parchment paper, leaving an overhang for easy lifting.
02 - Combine graham cracker crumbs, sugar, and melted butter in a bowl. Press evenly into the prepared pan. Bake for 8 minutes. Let cool slightly.
03 - In a large bowl, beat cream cheese and sugar until smooth. Add eggs one at a time, mixing well. Add sour cream, vanilla, flour, and salt; blend until just combined. Pour over the cooled crust.
04 - Toss diced apples with brown sugar, cinnamon, nutmeg, and lemon juice. Scatter evenly over the cheesecake layer.
05 - In a bowl, mix flour, oats, brown sugar, cinnamon, and salt. Cut in the cold butter with a pastry cutter or fingers until coarse crumbs form. Sprinkle evenly over the apples.
06 - Bake for 40–45 minutes, until the center is almost set. Cool completely at room temperature.
07 - Chill in the refrigerator for at least 2 hours before slicing. Lift bars from the pan using parchment. Cut into 16 squares and serve chilled or at room temperature.