Preheat oven to 180 C / Gas 4. Spray or grease one 23x33cm or similar sized tin.
Mix together the flour, oats, dark brown soft sugar and butter. Press into tin; bake for 20 minutes or until golden brown.
In a large deep metal bowl, beat egg whites until soft peaks form.
Blend together the pumpkin puree, evaporated milk, egg whites, sugar, salt, cinnamon, ginger and cloves. Pour into baked pastry case; bake for 30 minutes or until firm.
Mix together the chopped nuts, dark brown soft sugar and butter. Sprinkle topping on custard and bake additional 15 minutes. Remove from oven and allow to cool. Cut into squares and top with whipped cream, if desired.