To make an apple crumble, slice apples, and mix the apples with sugar, cinnamon, flour and lemon juice.Top the apples with a crumble mixture that contains oats, flour, cinnamon, brown sugar, butter and chopped nuts, and bake the crumble for 45 minutes at 350 degrees Fahrenheit.
Peel and core four large Golden Delicious apples, and cut the apples into 1/4 inch slices. Place the apple slices in a bowl, and toss the apples with 1/4 cup of sugar, 2 tablespoons of all-purpose flour, the juice of 1 lemon, and 1/2 teaspoon of ground cinnamon. Place the apple mixture in a 9x13 baking dish, and spread the apples in an even layer. Set the apples aside, and make the topping.
To make the topping, combine 1 cup of chopped pecans or walnuts, 1 cup of all-purpose flour, 1 1/4 cup rolled oats, 1/2 cup dark brown sugar, 1 teaspoon of cinnamon and a pinch of salt. Use a pastry cutter to roughly incorporate 1 stick of chilled unsalted butter into the topping mixture. The mixture should have pea-sized pieces after incorporating the butter.
Spoon the topping over the apples in an even layer, and bake the crumble at 350 degrees Fahrenheit for about 45 minutes, or until the crumble is browned and bubbling.