Toss apples with lemon juice and water in a shallow 2 litre baking dish.
In a large bowl, combine flour, oats, sugar, nutmeg and salt. Mix to blend thoroughly. Rub in the butter until mixture resembles coarse crumbs. Mix in walnuts. Crumble over apples to cover completely.
Bake for 40 to 45 minutes until top is lightly browned. Serve warm or at room temperature.
