1/4cup unsalted butter, chilled and cut into small pieces
1/3cup chopped walnuts
1/2cup granulated sugar
1/4teaspoon ground nutmeg
To prepare streusel, combine flour and both sugars in a large bowl. Cut in butter with fingers or pastry blender to form a streusel. Add walnuts and toss to combine. Refrigerate.
Preheat oven to 425F. Place pastry in pie pan; crimp edges.
To prepare filling, combine berries, sugar, cornstarch and nutmeg in a large bowl. Mix with a spoon and transfer to prepared pastry. Sprinkle with streusel. Brush pastry edges with milk. Bake 10 minutes. Reduce temperature to 350F and bake 35 minutes or until streusel is golden brown and juices are bubbling. Check after 20 minutes; if edges are browning too quickly, cover with strips of foil. Let cool completely on wire rack.