Position an oven rack in the center of the oven and preheat to 350 degrees F.
Peel, core, and slice apples into 1/4-inch slices. Place apples in a large mixing bowl and toss with sugar, lemon juice, flour, and cinnamon. Pour into a lightly greased 9 by 13-inch baking dish, and spread out into an even layer. Set aside.
In another large bowl, mix together the nuts, flour, oats, sugar, cinnamon, and salt for the topping. Using a pastry cutter or your fingers, gently work in the cold butter until pea-sized lumps are formed.
Top apples evenly with mixture and bake until apples are bubbly and topping is golden brown, about 45 minutes, rotating once halfway through cooking.
Serve warm or at room temperature with vanilla ice cream.