Thaw berries. Drain and reserve 1/2 cup of the juices; discard remaining juices. In a large bowl combine berries, sugar, reserved juices and cornstarch. Mix well and transfer to a 2-quart rectangular baking dish.
For topping, in a medium bowl combine flour and brown sugar. Using a pastry blender, cut in butter until pieces are pea size. Stir in oats and nuts. Sprinkle over berry mixture.
Bake in a 350 degrees F oven for 50 to 55 minutes or until filling is bubbly and topping is golden brown. Serve warm.