01 - Preheat oven to 350°F. Line a 9x13-inch baking pan with parchment paper, ensuring overhang on sides for easy removal.
02 - Whisk together flour, oats, sugar, salt, and cinnamon in a large bowl. Cut in cold butter using a pastry blender or fingertips until mixture resembles coarse crumbs.
03 - Press two-thirds of the crumble mixture firmly and evenly into the bottom of the prepared pan. Set aside remaining one-third for topping.
04 - Beat softened cream cheese and sugar until smooth and creamy. Add egg and vanilla extract, beating until fully incorporated and no lumps remain.
05 - Spread the cream cheese mixture evenly over the pressed crumble base using an offset spatula or back of a spoon.
06 - Toss diced peaches with cornstarch, lemon juice, and sugar in a medium bowl until peaches are evenly coated.
07 - Arrange the peach mixture evenly over the cream cheese layer, ensuring full coverage.
08 - Sprinkle the reserved crumble mixture evenly over the peach layer, covering completely.
09 - Bake for 38-42 minutes until top is golden brown and center appears set when gently shaken.
10 - Cool completely in pan on wire rack, then refrigerate for at least 2 hours to set completely. Slice into 12 bars and serve chilled or at room temperature.