Heat oven to 180C/fan 160C/gas 4. Thickly slice the bananas into a wide, shallow dish (a 20cm circular dish works well). Scatter the raspberries over the top.
Put the biscuits into a sturdy bowl or old saucepan and bash with the end of a rolling pin until reduced to crumbs. Stir in the sugar, then the butter, making sure all the crumbs are well coated. Scatter evenly over the fruit.
Bake for 20-25 mins, until just beginning to colour. Leave to stand for at least 10 mins for the topping to crisp up: you could even cook it just before eating the main course and let it wait in a warm place until you’ve finished. Serve with cream or custard.