Preheat oven to 375°F. Peel and core apples; cut into 1/4-inch thick wedges. Toss apples with lemon juice, in large bowl. Combine granulated sugar, cornstarch and 2 teaspoons cinnamon in small bowl; mix well. Add to apple mixture; mix well. Transfer to shallow 1-1/2 quart casserole or 11X7-inch baking dish.

For topping, combine brown sugar, flour, oats, remaining 1 teaspoon cinnamon and salt; mix well. Cut in butter with pastry blender or two knives until mixture resembles coarse crumbs. Stir in walnuts; sprinkle over apple mixture.

Bake 40 to 45 minutes or until topping is golden brown and apples are tender. Cool on wire rack at least 25 minutes before serving. Serve warm or at room temperature with ice cream.