Preheat oven: Set to 375°F (190°C) and line a baking sheet with parchment paper.
Make the crust: In a bowl, combine flour, cornmeal, and salt. Cut in the cold butter until the mixture becomes crumbly. Add cold water gradually and mix until the dough comes together. Form into a ball, flatten into a disk, and roll into a 10–12-inch circle.
Prepare the filling: Toss sliced strawberries with sugar and lemon juice.
Assemble: Place the dough on the baking sheet. Spoon the strawberry mixture into the center, leaving a 2-inch border. Fold the edges over the fruit, overlapping slightly for a rustic look. Brush the crust with beaten egg.
Bake: Bake for 35–40 minutes until the crust turns golden and the strawberries are bubbling.
Cool and serve: Let cool slightly, then dust with powdered sugar before serving.
