To make the crumble topping, sift the ﬂour into a mixing bowl and rub in the butter until the mixture resembles ﬁne breadcrumbs. Stir in the oats, ground almonds and sugar.
Preheat the oven to 190°C. Put the apples in a saucepan with the sugar and water. Cover and cook gently for 10 minutes, or until they begin to soften. Remove from the heat.
Peel and slice the banana and stir it into the apples, stir in the blackberries, then transfer to a 5-cup ovenproof dish and stand the dish on a baking tray.
Spoon the crumble mixture evenly over the fruit and press it down very lightly. Bake in the centre of the oven for 30–35 minutes, or until the crumble is golden brown and the juices from the ﬁlling begin to bubble up.
Apple and Rhubarb Crumble: Peel, core and thinly slice 2 cooking apples and toss with 2 cups of chopped rhubarb and 1/2 cup caster sugar. Put the fruit in an ovenproof dish, cover with the crumble topping and cook as above.