Preheat the oven to 400°F (205°C) so it’s fully ready when the slab is assembled. Keep an eye on the temperature for even browning.

Unroll and stack 2 refrigerated pie crusts, then roll into a 17x12-inch rectangle and fit into a 15x10x1-inch pan, pressing firmly into the corners. The crust should sit flush so the filling won’t leak.

Mix the sliced fresh strawberries, fresh raspberries, 1 cup sugar, cornstarch, and lemon juice in a large bowl until well blended. Stop when the cornstarch looks evenly distributed and the mixture looks glossy.

Spoon the berry mixture into the crust-lined pan. Spread it out evenly to help the slab bake with uniform thickness.

Roll the remaining 2 refrigerated pie crusts into another 17x12-inch rectangle and place on top of the berry mixture, pinching the edges together to seal. Press the seam firmly to keep juices contained.

Cut several slits in the top crust and brush with milk, then sprinkle with the remaining sugar. Look for slit openings so steam can escape and bubbling filling can release.

Bake at 400°F for 40 to 45 minutes until deep golden brown. You should see active bubbling through the slits and a crisp, browned crust.

Cool for 2 hours before slicing to let the filling set. The slab should hold neat squares without the juices running.