HEAT oven to 425°F. Combine flour, brown sugar and sugar in large bowl; cut in butter with pastry blender or 2 knives until mixture resembles coarse crumbs. Stir in nuts.

RESERVE 1 1/2 cups crumb mixture; press remaining crumb mixture onto bottom of 13 x 9-inch baking dish. Spoon mincemeat over crust.

COMBINE remaining ingredients in small bowl; mix well. Pour over mincemeat. Top with reserved crumb mixture.

BAKE 15 minutes; reduce oven to 350°F. Bake an additional 40 minutes or until golden brown around edges. Cool. Cut into squares. Serve warm or at room temperature, topped with ice cream, if desired.