1 lemon, juiced
Peel, core and slice apples into 1/4 inch slices. Place apples in a mixing bowl and toss with the lemon juice, sugar, flour and cinnamon. Pour into a lightly buttered 9 by 13 baking dish and spread out evenly.
In another large bowl, mix together the nuts, flour, oats, sugar, cinnamon and salt for the topping. using your fingers work in the butter until pea-sized lumps are formed.
Top apples evenly with the mixture and bake for 45 minutes rotating the pan once while cooking.