Combine first 5 ingredients in a medium bowl. Cut in butter with a pastry blender until mixture is the size of peas. Stir in pecans, almonds, and walnuts; pinch mixture with fingers to form large lumps of streusel.
Stir together apple and next 5 ingredients. Spoon filling into a greased 13" x 9" baking dish. Sprinkle streusel topping evenly over filling.
Bake, uncovered, at 350° for 1 hour or until apples are tender and topping is golden. Remove from oven; let stand 10 to 15 minutes. Serve with vanilla ice cream or lightly sweetened whipped cream, if desired.