For the filling: Preheat oven to 350 degrees and grease a 2 1/2 quart casserole dish. In a medium mxing bowl mix together apple slices, lemon juice, cinnamon, and sugar, let sit for at least 5 minutes.
For the topping: In a small mixing bowl stir together flour, brown sugar, and cinnamon. Cut in butter until mixture forms small clumps. Stir in oats and nuts.
To assemble: Pour apples in prepared pan and top with topping, lightly press the topping on top of the apples.
Bake until topping is brown and the filling is bubbly, about 40 to 45 minutes. Serve warm with ice cream or whipped cream.