01 - Preheat the oven to 350°F. Line a 9x13-inch baking pan with parchment paper, allowing some overhang for easy removal.
02 - Combine flour, oats, sugar, salt, and cinnamon in a large bowl. Cut in the cold butter using a pastry blender or your fingertips until the mixture resembles coarse crumbs.
03 - Press two-thirds of the crumble mixture firmly into the bottom of the prepared pan to form an even base. Reserve the remaining one-third for the topping.
04 - Beat cream cheese and sugar until smooth. Add egg and vanilla, and beat until fully combined and creamy.
05 - Spread the cream cheese mixture evenly over the pressed crumb crust.
06 - Toss diced peaches with cornstarch, lemon juice, and sugar until well coated.
07 - Arrange the peach mixture evenly over the cream layer.
08 - Sprinkle the reserved crumble mixture evenly over the peaches. Bake for 38-42 minutes, or until the top is golden and the center is just set.
09 - Allow to cool completely at room temperature, then refrigerate for at least 2 hours before slicing.
10 - Slice into 12 bars and serve chilled or at room temperature.