Preheat oven to 375 degrees. Combine the sugar, cornstarch, and salt in a bowl. Toss the berries and mint gently in mixture.
Spoon into the unbaked pie crust. Dot with the butter. Moisten pastry edge with water. Roll top crust 3 cm larger then the bottom crust and place over the filling, pressing edges to seal. Flute and cut slits or designs in the top of crust or prick the top with fork.
Chill half hour if necessary. Brush crust with the glaze and bake pie for 40 to 50 minutes. Serve slightly warm or at room temperature.
To order Nathalie Dupree Cooks Everyday Meal from a Well-Stocked Pantry