01 - Preheat oven to 350°F. Line a 9x13-inch baking pan with parchment paper, allowing overhang on sides for easy removal.
02 - Combine 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. Reserve remaining one-third for topping.
04 - Beat cream cheese and sugar until smooth and creamy. Add egg and vanilla, 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 bowl until peaches are evenly coated.
07 - Arrange the seasoned peach pieces evenly over the cream cheese layer.
08 - Sprinkle reserved crumble mixture evenly over peaches. Bake for 38-42 minutes until top is golden brown and center is just set.
09 - Let bars cool completely to room temperature. Refrigerate for at least 2 hours until firm before slicing into 12 bars.
10 - Slice into squares and serve chilled or at room temperature. Store refrigerated in an airtight container for up to 4 days.