Heat oven to 190C/fan 170C/gas 5. Lightly butter a 1.7 litre ovenproof shallow dish. Spread blackberries and drained peach slices in the bottom of the dish. Mix together the caster sugar and cinnamon, then sprinkle over the top and pour over the lemon juice and the reserved juice from the peaches. Bake, uncovered, for 10 mins until the juices begin to run. Remove the dish from the oven and set aside.
To make the topping, sift the polenta, flour, baking powder and a pinch of salt into a bowl. Rub in the butter with your fingertips until the mixture is the consistency of breadcrumbs. Add the sugar and toasted nuts and mix well, then stir in the milk to make a soft, sticky dough.
Increase oven temperature to 220C/fan 200C/gas 7. Drop spoonfuls of the dough over the top of the berries, then return to the oven for 15 mins until the topping is golden brown. Serve hot with cream, custard or ice cream.