First, add the all purpose flour, chopped walnuts, brown sugar, salt in a mixing bowl and mix well until combined.
Chop the cold butter into small pieces and add to the flour mixture. Use a pastry cutter or knife or fork or fingers and gently cut the butter into the flour mixture. Mix until the flour mixture looks like a coarse sand. Refrigerate until needed.
Wash, peel, core and slice the apples to about ⅛ inches thick. Until all apples are done, you can keep the sliced apples in cold water to prevent colorization. Add the apple slices to a large mixing bowl. Add the sugar, lemon juice, corn starch and ground cinnamon to the large bowl. Mix well until all dry ingredients are coated uniformly over the apple slices.
Pre heat the oven to 350 F. Transfer the apple mixture to a greased baking dish. Spread it out as an even layer. Add the crumble on top of the apple layer and spread it out evenly.
Bake for 45 mins in the lower rack of the preheated oven until the apple slices are nicely cooked and the top is golden brown.
Leave it to rest for 10 mins and optionally enjoy with a scoop of vanilla ice cream or whipped cream or even caramel sauce