For the filling, in a large frying pan add the rhubarb, balsamic vinegar and sugar and heat on a really high heat until just slightly softened. Remove the pan from the heat and transfer into a bowl.
Allow to cool and then add the berries and cornflour. Mix through and set aside.
Meanwhile, cube the butter and place in a mixing bowl with the flour. Rub together with your fingertips until it resembles breadcrumbs, then scrunch in the sugar to add a little texture.
Transfer the filling to a baking dish and sprinkle over the crumble topping.
Bake in the oven for 25 to 30 minutes, or until golden and bubbling. Delicious served with vanilla custard.
