Thickly slice the bananas into a wide, shallow dish. Scatter the raspberries over the top.
Put the biscuits in a sturdy bowl and bash with the end of a rolling pin until reduced to crumbs. Alternatively, put into a food processor and whizz until fine crumbs. Stir in the sugar and the butter, making sure it is well mixed. Scatter evenly over the fruit.
Bake for 20-25 minutes, until just beginning to colour. Leave to stand for 10 minutes for the topping to crisp up.