Heat the oven to 200C/180C fan/gas 6. Put the blackberries, cornflour, sugar and lemon juice into a 25cm round ovenproof dish. Mix a few times to dissolve the cornflour and coat the berries. Add the bay leaves, if using.
Put the crumble topping ingredients into a bowl and rub together using your fingertips, pressing the butter into the dry ingredients to get a chunky rubble. Scatter over the top of the berries and spread to cover. Bake for 40-45 mins or until the top is crisp and lightly golden and the berry juices are bubbling around the edges. Leave to cool slightly for 10 mins before serving.
