Preheat oven to 350°F. For the Mixed Berry Filling, toss berries with granulated sugar, lemon juice and vanilla in large bowl. Sprinkle with cornstarch; toss to coat well. Spoon into lightly greased 8- or 9-inch square baking dish.
For the Streusel Topping, mix flour, oats, brown sugar, cinnamon and ginger in medium bowl. Cut in butter with pastry blender or 2 knives until coarse crumbs form. Sprinkle evenly over fruit.
Bake 40 to 45 minutes or until fruit is tender and topping is golden brown. Serve warm with ice cream or whipped cream, if desired.