Preheat the oven to 375F and line a 9x13 pan with parchment paper, leaving overhang for lifting. Set aside while you mix the crumbs.

Combine all-purpose flour, granulated sugar, baking powder, and salt in a bowl. Add cold unsalted butter cubed and the egg, then mix until the mixture looks crumbly with no large wet patches (about 1-2 minutes).

Press two-thirds of the crumble mixture firmly into the prepared pan to form an even base. Press with your fingertips or the bottom of a measuring cup until compact and flat (visual cue: no loose crumbs).

Toss diced fresh peaches and fresh raspberries with the remaining granulated sugar, cornstarch, and lemon juice. Spread the fruit evenly over the base to the edges, so filling reaches near the pan corners (visual cue: a level fruit layer).

Scatter the remaining crumble evenly over the fruit filling in an even sandy layer. Cover most of the fruit while still allowing a few spots to peek through.

Bake at 375F for 35-40 minutes, until the topping is golden and the filling is bubbling at the edges. Rotate the pan once halfway through if your oven browns unevenly (visual cue: amber juices around the perimeter).

Cool completely in the pan before lifting out and cutting into bars. Wait until the center feels set and the topping looks dry, not glossy, to get clean slices.