A good apple crumble is both a useful and extremely popular family pudding, but if badly made it can have an uncooked, stodgy taste and texture. This dish is like a crumble, but the topping has chopped walnuts in it, making it more crunchy, and I think better, than the conventional crumble.
Preheat the oven. Peel and core the apples, and cut into smallish chunks. Peel the bananas and slice. Mix the apple and banana together in a bowl with the lemon juice and caster sugar and then spread in a rectangular or round ovenproof dish, about 2–3 pints (1.2–1.8 litres) in capacity.
To make the topping, gently melt the butter or margarine. Put the other ingredients in a bowl and sti